PyType_GetModule() may return wrong or unexpected result when a type has 
the feature flag Py_TPFLAGS_BASETYPE. The base type flag permits subclassing of an extension class. The function will return NULL when a type is subclasses in Python space or a different module object when the type is subclasses and initialized in a differen C extension module.

All subclassable types must use _PyType_GetModuleByDef() to safely access  module and module state struct. It's easy to miss the problem. I missed it when I ported some of my code to heap types and multiphase init.

I propose to either a guard to PyType_GetModule() to prevent misuse of the function or to add a variant of the function that prevents misuse.