Don't depend on argparse from pip for python >= 3.2

[tylerprete]

Argparse has been an included module in python since version 3.2 (https://docs.python.org/3/library/argparse.html).
It would be nice to stop requiring it to be installed for modern python versions.

pip has a way to specify that it's only a requirement for specific python versions:
argparse ; python_version <= "3.1"

[brendan-p-lynch]

Merged in the PR that should fix this. Will close once we do a release.