[ci] Bump Python to 3.12

[emaxx-google]

This changes the Python 3 version used in Continuous Integration in Github Actions.

The 3.12 version end-of-life is more than 4 years from now, unlike 3.9 which had only about a year left.

[github-actions[bot]]

Continuous Integration: All tests passed. C/C++ test coverage: 73.34% lines.