This is essentially the same problem as that reported in 
bug 494904 for pickle: deepcopy should treat instances 
of custom metaclasses the same way it treats 
instances of type 'type'.  I've attached a provisional fix 
which is basically a copy of the patch made to pickle (it 
checks to see if the type of the thing being deepcopied 
is a subclass of type).

One question: it seems to me that the exception 
handling code is unnecessary both here and in the 
pickle module.  In both cases, the first parameter to 
subclass is the result of a call to 'type' and the second 
is type 'type' itself, so it doesn't seem like there's any 
reason to worry about a TypeError.

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I changed the patch so that issubclass is called before the 
attempt to access __deepcopy__ (to avoid unbound instance 
method problem methioned in 494904).

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Thanks; I've checked this in. For the reason of the except
clause, see python.org/sf/502085.