gh-98627: Add an Optional Check for Extension Module Subinterpreter Compatibility

[ericsnowcurrently]

Enforcing (optionally) the restriction set by PEP 489 makes sense. Furthermore, this sets the stage for a potential restriction related to a per-interpreter GIL.

This change includes the following:

• add tests for extension module subinterpreter compatibility
• add _PyInterpreterConfig.check_multi_interp_extensions
• add Py_RTFLAGS_MULTI_INTERP_EXTENSIONS
• add _PyImport_CheckSubinterpIncompatibleExtensionAllowed()
• fail iff the module does not implement multi-phase init and the current interpreter is configured to check

• Issue: Add a Check for Extensions that Support Running in Subinterpreters #98627

[erlend-aasland]

FYI, converting _testinternalcapi.c to multi-phase init resolves four of the test failures.

test_configured_settings (test.test_capi.test_misc.SubinterpreterTest.test_configured_settings)
The config with which an interpreter is created corresponds ... Traceback (most recent call last):
  File "<string>", line 2, in <module>
ImportError: module _testinternalcapi does not support loading in subinterpreters
[...]

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test_configured_settings (test.test_capi.test_misc.SubinterpreterTest.test_configured_settings)
The config with which an interpreter is created corresponds ... ok
[...]