Add support for free-threaded Python 3.14t

[cclauss]

% uvx --with="valkey[libvalkey]" python3.14t -c "import valkey"

Installed 2 packages in 5ms
<frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to
    load module 'libvalkey.libvalkey', which has not declared that it can run safely without the GIL.
    To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.

• Test Python thread safety with pytest-run-parallel #42