remove python 2.6 trove classifier
remove python 2.6 trove classifier

OpenStack projects are no longer being tested under Python 2.6, so
remove the trove classifier implying that this project supports 2.6.

Change-Id: I9331afc5359023313db86f7e51cb1c081d19c65f

openstack/python-group-based-policy-client,noironetworks/python-group-based-policy-client,noironetworks/python-group-based-policy-client,stackforge/python-group-based-policy-client,stackforge/python-group-based-policy-client,openstack/python-group-based-policy-client