Add more checks on the GIL

Issue #10915, #15751, #26558:

* PyGILState_Check() now returns 1 (success) before the creation of the GIL and
  after the destruction of the GIL. It allows to use the function early in
  Python initialization and late in Python finalization.
* Add a flag to disable PyGILState_Check(). Disable PyGILState_Check() when
  Py_NewInterpreter() is called
* Add assert(PyGILState_Check()) to: _Py_dup(), _Py_fstat(), _Py_read()
  and _Py_write()

Parser/pgenmain.c

@@ -37,6 +37,14 @@ Py_Exit(int sts)
     exit(sts);
 }
 
+#ifdef WITH_THREAD
+/* Needed by obmalloc.c */
+int PyGILState_Check(void)
+{
+    return 1;
+}
+#endif
+
 int
 main(int argc, char **argv)
 {