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.

1 files changed, 2 insertions, 6 deletions

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