Python 3.15

[notatallshaw]

It would be good to add Python 3.15 to CI immediately, or at least prior to pip 26.1.

After, it would be good to add official support for Python 3.15 for pip 26.2, as that it the version that will likely be bundled with the official CPython 3.15 installers.

[pfmoore]

Should we just add GIL-enabled 3.15, or should we also test free-threaded? (I don't think pip does anything that might cause problems with free threading, but maybe we should explicitly check?)

[notatallshaw]

I think we should also check a free threaded version of Python, but as a separate issue, while free threading is marked experimental we need to decide whether to test or support the last major release, the latest prerelease, all releases, on one or all platforms, or some combination.

[notatallshaw]

Made separate issue: #13840