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.

chore: add type-hints to gitlab/v4/objects/merge_request_approvals.py

chore: check setup.py with mypy

Prior commit 06184daafd5010ba40bb39a0768540b7e98bd171 fixed the
type-hints for setup.py. But missed removing 'setup' from the exclude
list in pyproject.toml for mypy checks.

Remove 'setup' from the exclude list in pyproject.toml from mypy checks.

Ensure get() methods have correct 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.

chore: add type-hints to setup.py and check with mypy