Issue #12149: Update the method cache after a type's dictionnary gets

Issue #12149: Update the method cache after a type's dictionnary gets
cleared by the garbage collector.  This fixes a segfault when an instance
and its type get caught in a reference cycle, and the instance's
deallocator calls one of the methods on the type (e.g. when subclassing
IOBase).

Diagnosis and patch by Davide Rizzo.