bpo-39465: Use _PyInterpreterState_GET() (GH-20788)

Replace _PyThreadState_GET() with _PyInterpreterState_GET() in:

* get_small_int()
* gcmodule.c: add also get_gc_state() function
* _PyTrash_deposit_object()
* _PyTrash_destroy_chain()
* warnings_get_state()
* Py_GetRecursionLimit()

Cleanup listnode.c: add 'parser' variable.