Fix unittest2 for python 3.9
    
    Until we can retire python-unittest2 from fedora, let's make it work
    with python-3.9. For that we need to just use MutableMapping from
    collections.abc instead of collections as abstract base classes have
    been removed from collections [1].
    
    [1] https://docs.python.org/3.9/whatsnew/3.9.html