feat: enable future-compatible deployments

[2bndy5]

Free-threaded builds are exempt from abi3 builds.

• ref Support Python 3.14 and the free-threaded build #1418
• resolves New Python versions always require a pygit2 upgrade #1046

Why?

1. Reduces CI build time by only building 1 binary wheel for each architecture (and Linux-specific libc variants). This way, wheels are not built separately for each version of python, just the minimum supported python version.
2. Allows consumers of pygit2 to test their software with pre-releases of Python (excluding free-threaded variants).

Note

Until free-threaded variants supports CPytyhon's Limited API (AKA "abi3" wheels), each new release of python still mandates new release assets of pygit2 for free-threaded python versions (which can be retro-actively uploaded to PyPI if desired).

[2bndy5]

@jdavid I'm also working on a solution to build windows distributions with cibuildwheel from the current .github/workflows/wheels.yml workflow (includes x86, x64, and arm64 builds -- see my test-run CI summary). It would replace the current appveyor workflow. But, I would like to get this in first since it reduces the CI runtime.