Revert "update to version 0.7.1"
    
    This reverts commit 8dea6fc88a57c2455ab4724201c7beb9e2c07ec2.
    
    contrary to the docs,
    0.7.1 is not in fact compat with sqlalchemy 0.5 as the build
    % check fails with lots of instances of:
    
    File ".../migrate/versioning/schemadiff.py", line 72, in __init__
        self.affinity_A = self.type_A._type_affinity
    AttributeError: 'Text' object has no attribute '_type_affinity'
    
    I presume _type_affinity was introduced to sqlalchemy sometime
    after the version currently in RHEL.