[3.7] bpo-28866: No type cache for types with specialized mro, invalidation is hard...

[3.7] bpo-28866: No type cache for types with specialized mro, invalidation is hard. (GH-13157) (GH-13589)

* No type cache for types with specialized mro, invalidation is hard.

* FIX: Don't disable method cache custom types that do not implement mro().

* fixing implem.

* Avoid storing error flags, also decref.

* news entry

* Clear as soon as we're getting an error.

* FIX: Reference leak.
(cherry picked from commit 180dc1b0f4a57c3f66351568ae8488fa8576d7f0)

Co-authored-by: Julien Palard <julien@palard.fr>
https://bugs.python.org/issue28866