gh-141780: Make PyModule_FromSlotsAndSpec enable GIL if needed

[encukou]

We don't need to enable GIL for the PyModExport_ function, which is supposed to do as little as possible before returning a static array. Py_MOD_GIL_USED can be handled at module creation time, without enabling it transiently first.

The main difference from PyModuleDef (i.e. import.c's import_run_extension) is that he Py_mod_create function runs without the GIL enabled transiently. (It does run with GIL enabled permanently if Py_MOD_GIL_USED is specified -- same as the Py_mod_exec function, which always runs after the EnableGILTransient block.)

Also, if Py_mod_create makes a non-module object, Py_mod_gil is honored.

@colesbury, does this look OK?
(The first commit has the change; the next three 7 are tests/docs)

• Issue: Forcing enabling the GIL doesn't seem to work with PEP 793 initialization #141780

[encukou]

_testsinglephase is unusable in subinterpreters.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 5b8b87c 🤖

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https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F141785%2Fmerge

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[colesbury]

LGTM