Draft: Add python 3.14 free-threading for testing purpose

This branch is just to start playing with free-threading python.

gevent isn't ready for free-threading. Will require greenlet first. Issue to follow: https://github.com/python-greenlet/greenlet/issues/423 I did a hack to run the test without Gevent by default.

gcovr due to lxml and cppcheck aren't available either.

Nice guide: https://py-free-threading.github.io/porting-extensions/

Basic CI in place. Only one test failing so far.