Add python version constraint python>=2.7
Add python version constraint python>=2.7

The 'swift' command from v3.0.0 does not work in Python 2.6,
bacause some code is incompatible with Python 2.6
This patch is to add a constraint of python version

Change-Id: I5197cba0c2cd3135d08498df827a52f8bba98d4d
Closes-bug: #1590334

sohonetlabs/python-swiftclient,openstack/python-swiftclient,openstack/python-swiftclient,sohonetlabs/python-swiftclient