fix: disable GIL for free-threaded builds

[2bndy5]

This properly disables the GIL for free threaded python builds. Thanks to advice from @ngoldbaum

This patch simply gets pygit2 ready for use with Python 3.14t without warnings/errors.
All tests pass with Python v3.14t and python -c "import pygit2" completes without warnings.

I haven't done a through code review for spots (in src/*) where thread safety might be a concern.
My impression is that _pygit2 (when appropriate) invokes _libgit2 (which is compiled with cffi). So, I'm guessing most thread safety concerns are delegated to cffi.

[2bndy5]

I have another branch (2bndy5:test-with-cibuildwheel) that uses cibuildwheel to run unit tests on each built binary wheels. That branch is based on this branch, so my next PR depends on this PR.

[ngoldbaum]

Thanks for following up on this!!