> Update of the EXPERIMENTAL_ISOLATED_SUBINTERPRETERS status.

Also:

* _PyLong_Zero and _PyLong_One singletons are shared
* Py_None, Py_True and Py_False singletons are shared: bpo-39511 and PR 18301
* Static types like PyUnicode_Type and PyLong_Type are shared: see bpo-40077 and bpo-40601
* The dictionary of Unicode interned strings is shared: PR 20085
* context.c: _token_missing singleton is shared
* "struct _PyArg_Parser" generated by Argument Clinic is shared: see _PyArg_Fini()

Misc notes:

* init_interp_main(): if sys.warnoptions is not empty, "import warnings" is called to process these options, but not in subinterpreters: only in the main intepreter.
* _PyImport_FixupExtensionObject() contains code specific to the main interpreter. Maybe this function will not longer be needed once builtin extension modules will be converted to PEP 489 "multiphase initialization" API. I'm not sure.