[PATCH 2/3] Optimize the Python 3.8 refcount fix a tiny bit

[PATCH 2/3] Optimize the Python 3.8 refcount fix a tiny bit

This change uses the fact that our workaround to temporarily remove
the Py_TPFLAGS_METHOD_DESCRIPTOR flag uses the "mro" function
of PyType_Type, which never will change.
Therefore, the static keyword makes sure that this function lookup
happens only once.

Change-Id: I44b74556da1fac2596c81339af30cb66218276e2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Gbp-Pq: Name 0003-Optimize-the-Python-3.8-refcount-fix-a-tiny-bit.patch