Bump required python-cryptography version
    
    Since we started using 'Certificate.serial_number' instead of
    '.serial' from python-cryptography, bump the required version
    to the one where the above mentioned transition happened.
    
    https://fedorahosted.org/freeipa/ticket/6631
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>