Drop external requirement: argparse.

### Motivation:
`argparse` is in Python's [Standard Library](https://docs.python.org/2.7/library/argparse.html) since Python 2.7. Since we will be supporting only Python 2.7 onwards (see #619), there is no need to install `argparse` via `pip`.

One less external dependency; one more easily deployed project.

### Changelog:
* Remove `argparse` from project requirements.