Include "t" in ABI tag for `--disable-gil` builds

[colesbury]

Feature or enhancement

Context:

• PEP 703 (Build Configuration Changes)
• PEP 3149 – ABI version tagged .so files

The --disable-gil builds will have a different version specific ABI from the default CPython 3.13 builds due to reference counting differences and other changes. This should be indicated by a "t" (for "threading") in the ABI tag.

On POSIX, this is achieved by setting the ABIFLAGS variable, which feeds into the sysconfig EXT_SUFFIX and SOABI variables. The version specific shared libraries would look like:

• package.cpython-313t-darwin.so

On Windows, this is achieved by setting PYD_TAGGED_SUFFIX, which feeds into the sysconfig EXT_SUFFIX. The shared libraries using the version specific ABI would look like:

• _package.cp313t-win_amd64.pyd

Note that this does not address and is independent of the stable ABI. There's ongoing discussion about how best to support the stable ABI in --disable-gil builds.

cc @gpshead @brettcannon @vstinner

Linked PRs

• gh-109740: Use 't' in --disable-gil SOABI #109922

[vstinner]

Yep, I'm fine with "t" tag. Try to write a test to ensure that if Python is built with NoGIL, the tag is present. Maybe added it in test_sys which can check sys.abiflags.

[colesbury]

This is implemented, although there are still some follow-up tasks.