gh-98627: Add an Optional Check for Extension Module Subinterpreter Compatibility (gh-99040)

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

https://github.com/python/cpython/issues/98627

Lib/test/test_threading.py

@@ -1347,6 +1347,7 @@ class SubinterpThreadingTests(BaseTestCase):
                 allow_exec=True,
                 allow_threads={allowed},
                 allow_daemon_threads={daemon_allowed},
+                check_multi_interp_extensions=False,
             )
             """)
         with test.support.SuppressCrashReport():