unittest.TestCase is not multiple-inheritance cooperative

[jgrave3]

Bug report

Bug description:

unittest's TestCase does not call super().__init__() which breaks cooperative multiple inheritance.

I would be happy to add this, but I can imagine someone somewhere relies on cooperative multiple inheritance not working.

CPython versions tested on:

3.11, 3.12, 3.13, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-125252: Added support unittest.TestCase of multiple-inheritance co… #125287