Start testing Python 3.15

[cdce8p]

The first beta for Python 3.15 was released yesterday. Start running CI tests for it.

[cdce8p]

Some librt tests only compile individual c files. If Python.h isn't included first, these tests would raise ‘_POSIX_C_SOURCE’ redefined warnings on Python 3.15.

See https://bugzilla.redhat.com/show_bug.cgi?id=2416110 for a similar issue
https://docs.python.org/3/c-api/intro.html#include-files

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No need to backport anything specifically as building librt completely works fine already. It's just if only individual files are compiled.

[cdce8p]

Found a few more instances, so decided to create a separate PR after all. #21446

[cdce8p]

For 3.14 we added the classifier during the beta as well, once all tests passed. See #19199.

[hauntsaninja]

I'm okay with this, but it would be nice to make sure we support some of the headline features like lazy imports before we make a release with this classifier

[cdce8p]

Lazy imports do actually work already. At least with the "old" parser. See #20978 for more on that.

I'd be ok with removing the classifier for now if that's blocking the merge for you. We can always add it later as long as we don't forget to.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Great, thank you!

[hauntsaninja]

Cool, I filed #21447