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.

2 files changed, 11 insertions, 33 deletions

diff --git a/doc/rtd/topics/debugging.rst b/doc/rtd/topics/debugging.rst
index b897a318..a4a2779f 100644
--- a/doc/rtd/topics/debugging.rst
+++ b/doc/rtd/topics/debugging.rst
@@ -88,7 +88,7 @@ To quickly obtain a cloud-init log try using lxc on any ubuntu system:
 
 .. code-block:: shell-session
 
-    $ lxc init ubuntu-daily:xenial x1
+    $ lxc init ubuntu-daily:focal x1
     $ lxc start x1
     $ # Take lxc's cloud-init.log and pipe it to the analyzer
     $ lxc file pull x1/var/log/cloud-init.log - | cloud-init analyze dump -i -
@@ -104,13 +104,13 @@ To quickly analyze a KVM a cloud-init log:
 
 .. code-block:: shell-session
 
-    $ wget https://cloud-images.ubuntu.com/daily/server/xenial/current/xenial-server-cloudimg-amd64.img
+    $ wget https://cloud-images.ubuntu.com/daily/server/focal/current/focal-server-cloudimg-amd64.img
 
 2. Create a snapshot image to preserve the original cloud-image
 
 .. code-block:: shell-session
 
-    $ qemu-img create -b xenial-server-cloudimg-amd64.img -f qcow2 \
+    $ qemu-img create -b focal-server-cloudimg-amd64.img -f qcow2 \
     test-cloudinit.qcow2
 
 3. Create a seed image with metadata using `cloud-localds`
diff --git a/doc/rtd/topics/testing.rst b/doc/rtd/topics/testing.rst
index 7a1e3eec..5543c6f5 100644
--- a/doc/rtd/topics/testing.rst
+++ b/doc/rtd/topics/testing.rst
@@ -54,28 +54,22 @@ Test Layout
 * pytest tests should use bare ``assert`` statements, to take advantage
   of pytest's `assertion introspection`_
 
-  * For ``==`` and other commutative assertions, the expected value
-    should be placed before the value under test:
-    ``assert expected_value == function_under_test()``
-
-
 ``pytest`` Version Gotchas
 --------------------------
 
-As we still support Ubuntu 16.04 (Xenial Xerus), we can only use pytest
-features that are available in v2.8.7.  This is an inexhaustive list of
+As we still support Ubuntu 18.04 (Bionic Beaver), we can only use pytest
+features that are available in v3.3.2.  This is an inexhaustive list of
 ways in which this may catch you out:
 
-* Support for using ``yield`` in ``pytest.fixture`` functions was only
-  introduced in `pytest 3.0`_.  Such functions must instead use the
-  ``pytest.yield_fixture`` decorator.
-
 * Only the following built-in fixtures are available [#fixture-list]_:
 
   * ``cache``
   * ``capfd``
-  * ``caplog`` (provided by ``python3-pytest-catchlog`` on xenial)
+  * ``capfdbinary``
+  * ``caplog``
   * ``capsys``
+  * ``capsysbinary``
+  * ``doctest_namespace``
   * ``monkeypatch``
   * ``pytestconfig``
   * ``record_xml_property``
@@ -83,22 +77,6 @@ ways in which this may catch you out:
   * ``tmpdir_factory``
   * ``tmpdir``
 
-* On xenial, the objects returned by the ``tmpdir`` fixture cannot be
-  used where paths are required; they are rejected as invalid paths.
-  You must instead use their ``.strpath`` attribute.
-
-  * For example, instead of ``util.write_file(tmpdir.join("some_file"),
-    ...)``, you should write
-    ``util.write_file(tmpdir.join("some_file").strpath, ...)``.
-
-* The `pytest.param`_ function cannot be used. It was introduced in
-  pytest 3.1, which means it is not available on xenial.  The more
-  limited mechanism it replaced was removed in pytest 4.0, so is not
-  available in focal or later.  The only available alternatives are to
-  write mark-requiring test instances as completely separate tests,
-  without utilising parameterisation, or to apply the mark to the
-  entire parameterized test (and therefore every test instance).
-
 Mocking and Assertions
 ----------------------
 
@@ -168,9 +146,9 @@ Test Argument Ordering
 .. [#fixture-list] This list of fixtures (with markup) can be
    reproduced by running::
 
-     py.test-3 --fixtures -q | grep "^[^ -]" | grep -v '\(no\|capturelog\)' | sort | sed 's/.*/* ``\0``/'
+     python3 -m pytest  --fixtures -q | grep "^[^ -]" | grep -v 'no tests ran in' | sort | sed 's/ \[session scope\]//g;s/.*/* ``\0``/g'
 
-   in a xenial lxd container with python3-pytest-catchlog installed.
+   in an ubuntu lxd container with python3-pytest installed.
 
 .. _pytest: https://docs.pytest.org/
 .. _pytest fixtures: https://docs.pytest.org/en/latest/fixture.html