Fix monkeypatch.setattr() failing to undo patches to __bases__

[SemTiOne]

For class attributes not present in __dict__, preserve the pre-patch value so undo() can restore it with setattr(). Newly shadowed inherited attributes still use delattr(), preserving #156 behavior.

Added a regression test based on 5eaac19. Pre-fix:

>               delattr(obj, name)
E               TypeError: cannot delete '__bases__' attribute of immutable type 'ThingToTest'

src\_pytest\monkeypatch.py:437: TypeError
=========================== short test summary info ===========================
FAILED testing/test_monkeypatch.py::test_issue1938_patch_class_bases - TypeEr...
1 failed in 4.56s

Closes #1938.

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