Disable the GIL by default in free-threaded builds

[swtaarrs]

Feature or enhancement

Proposal:

Once #116167 is complete, users will be able to disable the GIL with PYTHON_GIL=0, but the GIL will remain enabled by default.

When we have enough confidence in the correctness of running without the GIL, we should change free-threaded builds to disable the GIL by default, as described in PEP 703.

This should be a very small code change, and is already detailed in this comment.

[corona10]

From my view, at least we should pass all unit tests with PYTHON_GIL=0

[colesbury]

Done in #118295