Introduce freethreading extra config option.

[ccoulombe]

For python 3.13+.
Disable the GIL and adapt the site_packages_path accordingly

Otherwise, _sanity_check_ebpythonprefixes fails with

== 2026-02-05 23:40:39,340 build_log.py:233 ERROR EasyBuild encountered an error: The Python install path (/home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python3.14/site-packages) was not added to sys.path (['', '/home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python314t.zip', '/home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python3.14t', '/home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python3.14t/lib-dynload', '/home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python3.14t/site-packages']) (at easybuild/easyblocks/p/python.py:804 in _sanity_check_ebpythonprefixes)

since /home/user/.local/easybuild/software/2023/x86-64-v3/Compiler/gcccore/python-freethreading/3.14.2/lib/python3.14/site-packages does not contains the abiflags (t).