Convert tests to use python3
    
    - drop or change shebang to python3
    - use $PYTHON when available, default to python3
    - convert print statements to the print() function
    - remove u"..." literals
    - remove Integer literals l and L
    - xml Element.getchildren() method is deprecated
    - sys.stdout.write -> sys.stdout.buffer.write
    
    https://pagure.io/certmonger/issue/138
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    With contributions from Stanislav Levin <slev@altlinux.org>