Remove 3.5 and xenial support (SC-711) (#1167)

Includes:
 - Update tox.ini and .travis.yml accordingly
 - Cleanup tox.ini with new tox syntax and cloud-init dependencies
 - Update documentation accordingly
 - Replace/remove xenial references where additional testing isn't required
 - Remove xenial checks in integration tests
 - Replace yield_fixture with fixture in pytest tests

Sections of code commented with lines like "Remove when Xenial is no
longer supported" still exist as they're require additional testing.

7 files changed, 8 insertions, 46 deletions

diff --git a/tests/integration_tests/bugs/test_gh626.py b/tests/integration_tests/bugs/test_gh626.py
index 7c720143..b80b677a 100644
--- a/tests/integration_tests/bugs/test_gh626.py
+++ b/tests/integration_tests/bugs/test_gh626.py
@@ -8,7 +8,6 @@ import pytest
 import yaml
 
 from tests.integration_tests import random_mac_address
-from tests.integration_tests.clouds import ImageSpecification
 from tests.integration_tests.instances import IntegrationInstance
 
 MAC_ADDRESS = random_mac_address()
@@ -29,7 +28,6 @@ iface eth0 inet dhcp
     ethernet-wol g"""
 
 
-@pytest.mark.sru_2020_11
 @pytest.mark.lxd_container
 @pytest.mark.lxd_vm
 @pytest.mark.lxd_config_dict(
@@ -39,11 +37,6 @@ iface eth0 inet dhcp
     }
 )
 def test_wakeonlan(client: IntegrationInstance):
-    if ImageSpecification.from_os_image().release == "xenial":
-        eni = client.execute("cat /etc/network/interfaces.d/50-cloud-init.cfg")
-        assert eni.endswith(EXPECTED_ENI_END)
-        return
-
     netplan_cfg = client.execute("cat /etc/netplan/50-cloud-init.yaml")
     netplan_yaml = yaml.safe_load(netplan_cfg)
     assert "wakeonlan" in netplan_yaml["network"]["ethernets"]["eth0"]
diff --git a/tests/integration_tests/bugs/test_lp1898997.py b/tests/integration_tests/bugs/test_lp1898997.py
index 115bd34f..d8ea54c3 100644
--- a/tests/integration_tests/bugs/test_lp1898997.py
+++ b/tests/integration_tests/bugs/test_lp1898997.py
@@ -47,8 +47,6 @@ version: 2
 @pytest.mark.lxd_vm
 @pytest.mark.lxd_use_exec
 @pytest.mark.not_bionic
-@pytest.mark.not_xenial
-@pytest.mark.sru_2020_11
 @pytest.mark.ubuntu
 class TestInterfaceListingWithOpenvSwitch:
     def test_ovs_member_interfaces_not_excluded(self, client):
diff --git a/tests/integration_tests/conftest.py b/tests/integration_tests/conftest.py
index b14b6ad0..2e44ef29 100644
--- a/tests/integration_tests/conftest.py
+++ b/tests/integration_tests/conftest.py
@@ -45,17 +45,6 @@ os_list = ["ubuntu"]
 
 session_start_time = datetime.datetime.now().strftime("%y%m%d%H%M%S")
 
-XENIAL_LXD_VM_EXEC_MSG = """\
-The default xenial images do not support `exec` for LXD VMs.
-
-Specify an image known to work using:
-
-    OS_IMAGE=<image id>::ubuntu::xenial
-
-You can re-run specifically tests that require this by passing `-m
-lxd_use_exec` to pytest.
-"""
-
 
 def pytest_runtest_setup(item):
     """Skip tests on unsupported clouds.
@@ -101,7 +90,7 @@ def disable_subp_usage(request):
     pass
 
 
-@pytest.yield_fixture(scope="session")
+@pytest.fixture(scope="session")
 def session_cloud():
     if integration_settings.PLATFORM not in platforms.keys():
         raise ValueError(
@@ -246,16 +235,6 @@ def _client(request, fixture_utils, session_cloud: IntegrationCloud):
     if lxd_use_exec is not None:
         if not isinstance(session_cloud, _LxdIntegrationCloud):
             pytest.skip("lxd_use_exec requires LXD")
-        if isinstance(session_cloud, LxdVmCloud):
-            image_spec = ImageSpecification.from_os_image()
-            if image_spec.release == image_spec.image_id == "xenial":
-                # Why fail instead of skip?  We expect that skipped tests will
-                # be run in a different one of our usual battery of test runs
-                # (e.g. LXD-only tests are skipped on EC2 but will run in our
-                # normal LXD test runs).  This is not true of this test: it
-                # can't run in our usual xenial LXD VM test run, and it may not
-                # run anywhere else.  A failure flags up this discrepancy.
-                pytest.fail(XENIAL_LXD_VM_EXEC_MSG)
         launch_kwargs["execute_via_ssh"] = False
     local_launch_kwargs = {}
     if lxd_setup is not None:
@@ -276,21 +255,21 @@ def _client(request, fixture_utils, session_cloud: IntegrationCloud):
         _collect_logs(instance, request.node.nodeid, test_failed)
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def client(request, fixture_utils, session_cloud, setup_image):
     """Provide a client that runs for every test."""
     with _client(request, fixture_utils, session_cloud) as client:
         yield client
 
 
-@pytest.yield_fixture(scope="module")
+@pytest.fixture(scope="module")
 def module_client(request, fixture_utils, session_cloud, setup_image):
     """Provide a client that runs once per module."""
     with _client(request, fixture_utils, session_cloud) as client:
         yield client
 
 
-@pytest.yield_fixture(scope="class")
+@pytest.fixture(scope="class")
 def class_client(request, fixture_utils, session_cloud, setup_image):
     """Provide a client that runs once per class."""
     with _client(request, fixture_utils, session_cloud) as client:
diff --git a/tests/integration_tests/datasources/test_lxd_discovery.py b/tests/integration_tests/datasources/test_lxd_discovery.py
index da010813..eb2a4cf2 100644
--- a/tests/integration_tests/datasources/test_lxd_discovery.py
+++ b/tests/integration_tests/datasources/test_lxd_discovery.py
@@ -59,13 +59,9 @@ def test_lxd_datasource_discovery(client: IntegrationInstance):
     )
     if (
         client.settings.PLATFORM == "lxd_vm"
-        and ImageSpecification.from_os_image().release
-        in (
-            "xenial",
-            "bionic",
-        )
+        and ImageSpecification.from_os_image().release == "bionic"
     ):
-        # pycloudlib injects user.vendor_data for lxd_vm on bionic and xenial
+        # pycloudlib injects user.vendor_data for lxd_vm on bionic
         # to start the lxd-agent.
         # https://github.com/canonical/pycloudlib/blob/main/pycloudlib/\
         #    lxd/defaults.py#L13-L27
diff --git a/tests/integration_tests/modules/test_disk_setup.py b/tests/integration_tests/modules/test_disk_setup.py
index 22277331..8f9d5f40 100644
--- a/tests/integration_tests/modules/test_disk_setup.py
+++ b/tests/integration_tests/modules/test_disk_setup.py
@@ -20,7 +20,7 @@ def setup_and_mount_lxd_disk(instance: LXDInstance):
     )
 
 
-@pytest.yield_fixture
+@pytest.fixture
 def create_disk():
     # 640k should be enough for anybody
     subp("dd if=/dev/zero of={} bs=1k count=640".format(DISK_PATH).split())
@@ -133,10 +133,9 @@ class TestPartProbeAvailability:
         assert sdb["children"][1]["name"] == "sdb2"
         assert sdb["children"][1]["mountpoint"] == "/mnt2"
 
-    # Not bionic or xenial because the LXD agent gets in the way of us
+    # Not bionic because the LXD agent gets in the way of us
     # changing the userdata
     @pytest.mark.not_bionic
-    @pytest.mark.not_xenial
     def test_disk_setup_when_mounted(
         self, create_disk, client: IntegrationInstance
     ):
diff --git a/tests/integration_tests/modules/test_lxd_bridge.py b/tests/integration_tests/modules/test_lxd_bridge.py
index 2cb3f4f3..3292a833 100644
--- a/tests/integration_tests/modules/test_lxd_bridge.py
+++ b/tests/integration_tests/modules/test_lxd_bridge.py
@@ -33,8 +33,6 @@ class TestLxdBridge:
         """Check that the expected LXD binaries are installed"""
         assert class_client.execute(["which", binary_name]).ok
 
-    @pytest.mark.not_xenial
-    @pytest.mark.sru_2020_11
     def test_bridge(self, class_client):
         """Check that the given bridge is configured"""
         cloud_init_log = class_client.read_from_file("/var/log/cloud-init.log")
diff --git a/tests/integration_tests/modules/test_users_groups.py b/tests/integration_tests/modules/test_users_groups.py
index 3d1358ce..fddff681 100644
--- a/tests/integration_tests/modules/test_users_groups.py
+++ b/tests/integration_tests/modules/test_users_groups.py
@@ -106,7 +106,6 @@ def test_sudoers_includedir(client: IntegrationInstance):
     https://github.com/canonical/cloud-init/pull/783
     """
     if ImageSpecification.from_os_image().release in [
-        "xenial",
         "bionic",
         "focal",
     ]: