Remove Python 2.7 support
    
    Currently, the only supported release that uses Python 2.7 is epel7
    which is ending by the end of June. Therefore I removing the support
    for this old Python version. Code will stay as it is for now, only
    modifying scripts and configurations for testing and other Python 2
    mentions. This also means that future code changes can break
    Python 2 compatibility.
    
    JIRA: RHELCMP-13775
    
    Signed-off-by: Ondřej Nosek <onosek@redhat.com>