Add a workaround for pytz and pip>=1.4
Add a workaround for pytz and pip>=1.4

pip versions >= 1.4 will not install pytz unless a minimum
version containing a letter is specified (this workaround is
taken from Babel Python library).

Change-Id: Ic013fca82816cd89e446282d3936e38ea2c39274

rcherrueau/sqlalchemy-migrate,rcherrueau/sqlalchemy-migrate,dannon/sqlalchemy-migrate,andras-tim/sqlalchemy-migrate,openstack/sqlalchemy-migrate,stackforge/sqlalchemy-migrate,openstack/sqlalchemy-migrate