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On further thought, here's a slightly improved version of the patch I posted above.
The difference is that the first version allowed through attempted __class__ assignments where either the old or new class was a subclass of ModuleType; the new version only allows through attempted assignments if both the old AND new class are a subclass of ModuleType.
In practice this doesn't make any difference, because the compatibility-checking code will reject any attempt to switch from a ModuleType subclass to a non ModuleType subclass or vice-versa. So both patches are correct. But the new patch is more obviously correct.