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The problem's easy to fix (using inspect.getmro instead of direct 
__mro__ access) but hard to reproduce -- what type not recorded in the 
dispatch table lacks an __mro__?  Can't find one easily, and I'd like to 
have a unit-test reproducing the bug before I commit the fix.

Vincenzo, can you explain what the type is happening to be in that value 
whose deepcopy gives problems?  or can anybody else suggest a type as 
above?