Support free-threaded Python

[rissson]

I was attempting to run a project using xmlsec with a free-threaded python build (3.14.7t with PYTHON_GIL=0), and it segfaults.

lxml is planning free-threaded support in 7.0.0 (https://bugs.launchpad.net/lxml/+bug/2111289) and the current beta (7.0.0b1) seems to be running fine with free-threading enabled.

It'd be great if xmlsec could also support free-threading.

[mxamin]

@rissson I'm currently working on a major release and probably I'll include free-threaded python in it too.