Add dnf support to critpath.py
    
    - made critpath.py python3 compliant
    - added support for dnf package resolution to critpath.py
    - use dnf for dependency solving if used on a release after Fedora 26
      or the script is run under Python 3
    - requires the DNF 2.0 library API
    - exits if the YUM version is not recent enough when using YUM
    - override the defaults for older releases by passing --dnf
    - maintain the old hack using a mapping of base architectures to
      architectures to use for package dependency solving
    
    Signed-off-by: Robert Marshall <rmarshall@redhat.com>