argparse was external in python 2.6 but not anymore, remove it from
This should help with pip 8.0 that gets confused in this situation.
Installation of the external argparse is not needed.
The configuration file parsers have a defined API, but we aren't
enforcing use of that. This adds an abstract base class that they
all will now inherit from. Additionally, new tests are added to
ensure the classes inherit from the new base class.
Moves tests for files in the config/ directory into their own
Fixes the test class name for the ToxIni tests.