Import ABCs from collections.abc
    
    Python 3 has moved all collection abstract base classes to
    collections.abc. Python 3.7 started to deprecate the old aliases.
    
    The whole import block needs to be protected with import-error and
    no-name-in-module, because Python 2 doesn't have collections.abc module and
    collections.abc.Mapping, while Python 3 doesn't have collections.Mapping.
    
    Fixes: https://pagure.io/freeipa/issue/7609
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>