Fix TypeError in autodoc mock _make_subclass for non-string name arguments

[piyushhhxyz]

Summary

Add defensive str() conversion in _make_subclass to handle cases where the name parameter is not a string (e.g. TypeVar, tuple of types). While __getitem__ already converts to str, _make_subclass can also be called from __new__ where the name argument may not be a string.

This prevents TypeError when building docs for generically-typed classes that use TypeVar subscripts on mocked objects.

Closes #7898

Changes

• sphinx/ext/autodoc/_dynamic/_mock.py: Wrap name with str() in _make_subclass for __display_name__, __name__, and the type() call

Testing

• Verified the existing test suite covers this scenario (test_MockObject at line 57-64 already tests TypeVar subscripting)
• Ran the full autodoc mock test suite on the base commit (ec9af606c) with the equivalent fix applied:

python3 -m pytest tests/test_ext_autodoc_mock.py -v

tests/test_ext_autodoc_mock.py::test_MockModule PASSED
tests/test_ext_autodoc_mock.py::test_MockObject PASSED
tests/test_ext_autodoc_mock.py::test_mock PASSED
tests/test_ext_autodoc_mock.py::test_mock_does_not_follow_upper_modules PASSED
tests/test_ext_autodoc_mock.py::test_abc_MockObject PASSED
tests/test_ext_autodoc_mock.py::test_mock_decorator PASSED
tests/test_ext_autodoc_mock.py::test_MockObject_getitem_with_TypeVar PASSED
======================== 7 passed ========================