fix(build): Add free-threaded python3.14t to the testing

[cclauss]

Let's drop Python's Global Interpreter Lock (GIL).

• https://py-free-threading.github.io
• https://www.python.org/downloads/release/python-3140/

gyp-next/pylib/gyp/input.py

Line 610 in cb13bbc

parallel_state.condition = threading.Condition()

Adding pytest-run-parallel and running pytest on Python >= 3.9 works, but adding the options below demonstrates that the use of global state makes this code thread-unsafe.
% pytest --iterations=8 # Fails.
% pytest --parallel-threads=auto # Fails.
% pytest --iterations=8 --parallel-threads=auto # Fails.