Tests/Docs: Update tests to Cython 0.29.30, mention in docs

[EwoutH]

Cython 0.29.30 is required for building Numpy with Python 3.11. This commit updates that in the core/tests/test_cython.py and random/tests/test_exending.py files.

It also mentions that Cython 0.29.30 is needed in the INSTALL documentation.

This PR is a small follow-up on #21514 and #21530. It might also be needed to be backported, to the 1.22.x maintenance branch, like #21525.

[mattip]

Thanks. Perhaps we should have a better way to sync all these places.

[charris]

Thanks @EwoutH . I'm going to omit backporting as tests pass and I don't plan more 1.22.x releases. 1.23.x will be the first to support Python 3.11.

[InessaPawson]

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[EwoutH]

Thanks for the kind words and the info, @InessaPawson!