Drop Python 2 support, use f-strings, drop Python 2 workarounds
    
    This officially drops Python 2 support, because I'm kinda done
    with dealing with Python 2. I'll try doing Python 3 builds for
    EPEL 7, if that doesn't work, I'll do branch releases from
    before this commit if I need to update EPEL 7 in future. This
    lets us import mock from unittest, drop __future__ imports, use
    f-strings everywhere (hope I got this conversion right...) and
    drop an annoying workaround for datetime.timestamp() not being
    in Python 2.x. Yay!
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>