Use `encoding="utf-8"` in `open()` and open-like functions.

https://peps.python.org/pep-0597/

Commit f0152dc3cc9a42aa4dc3c0014b4c29381e9b39d6 removed situation
where `get()` in a `GetWithoutIdMixin` based class could return `None`

Update the type-hints to no longer return `Optional` AKA `None`

Add a test to make sure that all of the `gitlab/v4/objects/` files are
imported in `gitlab/v4/objects/__init__.py`

Add the get() methods for the GetWithoutIdMixin based classes.

Update the tests/meta/test_ensure_type_hints.py tests to check to
ensure that the get methods are defined with the correct return type.

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