gh-120838: Disallow Py_Finalize() if Py_RunMain() is Running

[ericsnowcurrently]

Py_RunMain() is responsible for finalizing the runtime. Extension modules should never call Py_Finalize(). Enforcing this helps keep finalization conceptually simpler and helps get closer to making the Py_Finalize() implementation simpler in practice.

Note that the relevant docs change is part of gh-120839.

• Issue: Some Runtime Finalization Constraints Are Not Enforced Nor Documented #120838

[ericsnowcurrently]

@ncoghlan, any objections to this change?

[ericsnowcurrently]

CC @vstinner