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Regarding the macro definition, I would be fine with changing it to _hashlib_state.
The dance you have found inside the Init, makes shure that the very same module is returned if Init is called twice or multiple times, before the Module is unloaded. A month back, when I created this patch, I had statements such as test.import.import_fresh_module(...) call the Init-method multiple times, before a module was unloaded. This was apparently a bug, as I can no longer reproduce this behavior, but at that time I thought it was the expected behavior :-)
The last code snipped verifies, that we only dereference the type if the dealloc function is not being called from inside the subtype_dealloc function. This is necessary because the subtype_dealloc function itself contains a decref of the respective type object. Without this check, we would then end up decrefing the type too many times.