Support free-threaded CPython builds and upload cp313t wheels

[ngoldbaum]

Unfortunately actually doing this might end up being blocked on upstream support in CFFI for free-threaded Python, see python-cffi/cffi#126 for more detail.

We are working on fixing CFFI in a fork: https://github.com/quansight-labs/cffi.

It looks like I can build pygit2 if I install setuptools and CFFI from the fork, and then build with --no-build-isolation:

(.venv) ± C_INCLUDE_PATH=/opt/homebrew/include python -m pip install . --no-build-isolation
Processing /Users/goldbaum/Documents/pygit2
  Preparing metadata (pyproject.toml) ... done
Requirement already satisfied: cffi>=1.17.0 in /Users/goldbaum/Documents/free-threaded-compatibility/.venv/lib/python3.13t/site-packages (from pygit2==1.17.0) (1.18.0.dev0)
Requirement already satisfied: pycparser in /Users/goldbaum/Documents/free-threaded-compatibility/.venv/lib/python3.13t/site-packages (from cffi>=1.17.0->pygit2==1.17.0) (2.22)
Building wheels for collected packages: pygit2
  Building wheel for pygit2 (pyproject.toml) ... done
  Created wheel for pygit2: filename=pygit2-1.17.0-cp313-cp313t-macosx_15_0_arm64.whl size=261670 sha256=f3aa4e995b7a8f81d13040440b0fec75696e7407a15d9035ba7852b7c32871c9
  Stored in directory: /private/var/folders/nk/yds4mlh97kg9qdq745g715rw0000gn/T/pip-ephem-wheel-cache-5aoxf597/wheels/30/43/a7/885db85606d7de4f35d4eb2ecc0a8b6c24e6615ed88f7f3eb1
Successfully built pygit2
Installing collected packages: pygit2
Successfully installed pygit2-1.17.0

Not totally sure why I needed to pass C_INCLUDE_PATH to find libgit2 from homebrew.