> If sys.modules['_ast'] is cleared and then _ast is imported again, _PyState_AddModule() is called to store the new _ast module instance which calls astmodule_clear() on the old module instance.

I'm confused about this. In the repro below the sys.modules.clear() is indeed cleared, but references to all modules are retained. 

Where is the garbage collection/module re-initialization coming from? All of the references are live.