CPython 3.14 support

[vytas7]

Formalize CPython 3.14 support:

• Make the test suite pass on 3.14: chore: implement initial support for CPython 3.14 #2412.
• Add a basic CPython 3.14 CI gate: chore: implement initial support for CPython 3.14 #2412.
• Build CPython 3.14 wheels.
• Add CPython-cythonized and wheel tests to the CI.
• Advertise support using "trove classifiers".
• Check if anything needs an update in CONTRIBUTING.md.
• Update release notes regarding supported platforms.
• Check for any new warnings emitted when running tests, e.g., whether we are relying on any deprecated functionality that will be removed in future Python versions.
• Check if all the binary test dependencies (such as orjson) are ready.

Note: we probably don't want to formally advertise this using Trove classifiers at least until the first beta release (expected in May 2025) because things can change rapidly until then. For the binary wheels, we need to wait for rc1 (expected in the end of July 2025) or whenever the ABI compatibility with the final release is announced.