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

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.