compat: Python 3.9 no longer offers collections.Mutable* ABCs
    
    Solution is to move over onto collections.abc dedicated submodule
    that hosts these exclusively since 3.9.
    
    References:
    https://docs.python.org/3/whatsnew/3.3.html#collections
    https://docs.python.org/3/whatsnew/3.7.html#id3
    https://docs.python.org/3.7/library/collections.html#collections-container-datatypes
    https://docs.python.org/3.9/whatsnew/3.9.html#removed
    
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>