Add argparse requirement for py26
Add argparse requirement for py26

The argparse module was included in the python
standard library in python 2.7 or greater only.
This should allow bash8 to work on python 2.6 as
advertised.

Change-Id: I87f47a796de734f261b85bc72414a30d56926dcd

openstack-dev/bashate,openstack-dev/bashate