We have a couple dependencies that no longer work on older versions of

Python, which would make installation problematic for users and CI

maintenance more troublesome. We're also missing a dependency for Python

2.6 and lower.
This change adds the argparse dependency for Python 2.6 and lower, and

caps the paramiko dependency to the latest version for each Python

version supported. It also changes the mercurial dependency in

dev-requirements.txt to choose an appropriate version for each version

of Python, using version specifiers.

Manually tested installation with Python 2.6 and 2.7.