tests: Migrate tests helpers to Python3
    
    As it is commonly known, Python2 reaches its EOL in 2020.
    For this reason, many distributions massively drop Python2
    packages.
    
    Certmonger utilizes by itself only python-dbus and only at
    build time for testing. With some help of 2to3 utility
    several tests helpers have been migrated to Python3:
    - converts the print statement to the print() function (reindented)
    - remove u"..." literals
    - remove Integer literals l and L
    - change shebang to python3
    
    TODO: migrate examples of helpers.
    
    Fixes: https://pagure.io/certmonger/issue/138
    Signed-off-by: Stanislav Levin <slev@altlinux.org>