Add Python 3.14 free-threading build support

[ericbuehl]

With the release of Python 3.14, the new free-threaded (no-GIL) mode is becoming more widely used.

It would be great if mysqlclient could confirm compatibility with python3.14t (the free-threaded ABI). Without it, freethreaded users are met with the following message:

$ python
Python 3.14.0rc3 free-threading build (main, Sep 18 2025, 19:37:18) [Clang 20.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import MySQLdb
<frozen importlib._bootstrap>:491: RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'MySQLdb._mysql', 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.