[3.13] gh-142457: Support using 3.14 and 3.15 for generation scripts

[ZeroIntensity]

Check for available Python 3.14 and Python 3.15 installations in the configure script. This allows us to use the 3.14 version installed by actions/setup-python in our 3.13 CI, rather than falling back to a 3.12 copy baked into the image.

I've omitted the blurb entry for this one, because while this is technically a change to the build, these regeneration scripts usually aren't visible in practice. If anyone is strongly in favor of adding a news entry here, feel free to let me know.

• Issue: 3.13 CI uses 3.12 for generation scripts #142457

[ZeroIntensity]

For anyone interested, the precedent here is that we check for an installed 3.15 copy in 3.14's configure script:

cpython/configure.ac

Line 208 in d14697d

[python$PACKAGE_VERSION python3.15 python3.14 python3.13 python3.12 python3.11 python3.10 python3 python],

However, we don't do this for any other branches, at least as far as I can tell.

[ZeroIntensity]

cc @hugovk

[hugovk]

Noting this is similar to #137619 that only backported to 3.14.

We could mark this as a backport of that, and include the NEWS. Or try a backport of that. But I don't mind much either way :)