(I see some time has passed since the last message. I'm assuming the 
issue wasn't fixed, correct me if I'm wrong.)

Here's a use case for using deepcopy: I'm developing a simulations 
framework called GarlicSim, all Python. You can see a short video here: 
http://garlicsim.org/brief_introduction.html
The program handles world states in simulated worlds. Some simpacks 
deepcopy the existing world state to generate the next world state.

This bug is currently preventing me from writing simpacks whose world 
states reference instance methods, which is a severe limitation for me.

I think this issue should be bumped in priority. Also, in the mean time, 
a more informative error message should be given (Like "deepcopy can't 
handle instance methods."), instead of the current cryptic one.