Add typing-extensions to requirements-tests.txt

[Avasam]

From the following comment: #9434 (comment)

I think typing_extensions is only available here because mypy depends on it, and we have mypy in our requirements-tests.txt file. We should probably add it to requirements-tests.txt, since implicit dependencies are generally a bad idea, and we also need the dependency for e.g. stubtest_stdlib.py. Want to make another PR adding it to requirements-tests.txt?

[AlexWaygood]

Thanks!