A commonly reported issue from users on Gitter is that they get an
AttributeError for an attribute that should be present. This is often
caused due to the fact that they used the `list()` method to retrieve
the object and objects retrieved this way often only have a subset of
the full data.

Add more details in the AttributeError message that explains the
situation to users. This will hopefully allow them to resolve the
issue.

Update the FAQ in the docs to add a section discussing the issue.

Closes #1138

Add support for `projects.groups.list()` endpoint.

Closes #1717

Before we were generating 725 tests for the
meta/test_ensure_type_hints.py tests.  Which isn't a huge concern as
it was fairly fast. But when we had a failure we would usually get two
failures for each problem as the same test was being run multiple
times.

Changed it so that:
  1. Don't add tests that are not for *Manager classes
  2. Use a set so that we don't have duplicate tests.

After doing that our generated test count in
meta/test_ensure_type_hints.py went from 725 to 178 tests.

Additionally removed the parsing of `pyproject.toml` to generate files
to ignore as we have finished adding type-hints to all files in
gitlab/v4/objects/. This also means we no longer use the toml library
so remove installation of `types-toml`.

To determine the test count the following command was run:
  $ tox -e py39 -- -k test_ensure_type_hints

chore: enable mypy for tests/meta/*

The test was checking twice if the same group3 was not in the returned
list.  Should have been checking for group3 and group4.

Also added a test that only skipped one group and checked that the
group was not in the returned list and a non-skipped group was in the
list.

Fix classes which don't have correct 'get()' methods for classes
derived from GetMixin.

Add a unit test which verifies that classes have the correct return
type in their 'get()' method.

The 'test_mro.py' file is not really a unit test but more of a 'meta'
check on the validity of the code base.