Your graph is simply too deep for Python to handle it. The copy module
(and also the pickle) pushes a new stack frame every time a new
container object is encountered. Therefore, there is a recursion limit
to prevent the interpreter from crashing.

If you want, you can carefully increase the limit by using the function
`sys.setrecursionlimit`. Using your example:

   >>> sys.setrecursionlimit(2000)
   >>> g2 = copy.deepcopy(g)
   >>> # no error