Remove unnecessary check for __deepcopy__ argument

[dmoisset]

The implementation for QueryDict..__depcopy__ accepts an optional memo argument and check that it is not None. However, according to https://docs.python.org/3.6/library/copy.html the memo argument is mandatory and you're guaranteed to always get a dict (the copy.deepcopy stdlib function takes care of ensuring this) so the check is redundant.

[rixx]

Looks good. The deepcopy interface has not changed in any of the supported python versions (or forever, really).