DNM: enable the experimental JIT in 3.13+

[tianon]

This is for testing, not for merging.

See:

• https://docs.python.org/3.13/whatsnew/3.13.html#an-experimental-just-in-time-jit-compiler
• Is there a 3.13 nogil container? #947

Test it with something like:

$ docker build --pull https://github.com/docker-library/python.git#refs/pull/1043/merge:3.13/slim-trixie

[tianon]

Oof, there's a lot to unpack with this one.

• https://docs.python.org/3.13/whatsnew/3.13.html#an-experimental-just-in-time-jit-compiler mentions --experimental-jit-interpreter but it's actually --enable-experimental-jit as noted further down
• https://github.com/python/cpython/blob/3.13/Tools/jit/README.md has several other requirements, notably Python 3.11+ and LLVM version 18 (specifically and only 18: https://github.com/python/cpython/blob/3.13/Tools/jit/_llvm.py#L11)

That is going to be really limiting in our ability to enable this (for example, LLVM 18 isn't even available in Debian stable - it's currently in testing, but I don't know if it'll survive to release of trixie).

[tianon]

(and to head it off: compiling LLVM from source is very much out-of-the-question unless/until upstream decides this feature is part of a standard recommended install and should be enabled for ~everyone)

[tianon]

Rebased on #1042, applied more hacks; at least the slim-trixie builds here should succeed.

[tianon]

(unsurprisingly, it looks like Alpine is unsupported: build.py: error: argument target: invalid get_target value: 'x86_64-pc-linux-musl')