Python 3.15 compatibility

[pekkaklarck]

This issue covers:

• Run all tests with Python 3.15 preferably both on Linux and on Windows
• Fix possible problems. I don't expect there to be many/any. Some problems were reported in Upcoming Python 3.15 incompatibilities #5679, but changes causing them were in the end reverted.
• Set GitHub actions to run tests with Python 3.15.
• Update classifiers, README, etc.

[pekkaklarck]

I run tests on Windows and there were some failures. At least some of them seemed to be related to encodings. I don't have time to investigate them now, but that certainly needs to be done before the final release.