Fix tox.ini to not reference deleted testenv
Fix tox.ini to not reference deleted testenv

The pyflakes testenv was removed by commit 806a5b9.

Change-Id: If696c91ac013c806cf80805084045e0b153c5508

redhat-openstack/pbr,citrix-openstack-build/pbr,varunarya10/pbr,citrix-openstack-build/pbr,citrix-openstack-build/pbr,JioCloud/pbr,openstack-dev/pbr,openstack-dev/pbr,klmitch/pbr,WiFast/pbr,openstack-dev/pbr,varunarya10/pbr,JioCloud/pbr,klmitch/pbr,redhat-openstack/pbr,WiFast/pbr