Fix warning about returning tuples from `get_object_members()`

[AWhetter]

Running the tests outputs the following warning:

.tox/py312/lib/python3.12/site-packages/sphinx/ext/autodoc/__init__.py:818: RemovedInSphinx80Warning: Returning tuples of (name, object) as the second return value from get_object_members() is deprecated. Return ObjectMember(name, object) instances instead.
    for (mname, member, isattr) in self.filter_members(members, want_all):

We should address this before Sphinx v8 is released and this warning becomes an error.