cli: fix serializable() helper with new python3-munch
    
    The Munch(List(items=[])) structure isn't possible to copy().  The
    documentation for Munch.copy() says it does shallow copy (which doesn't
    seem to be truth anymore, otherwise they shouldn't call the constructor
    for sub-arrays).
    
    The problem is of course on our side, because it is terribly ugly to
    inherit from list() and change it's default behavior (the required items
    argument).
    
    Since we indeed want a shallow copy, doing just dict() should be enough.
    We loose the Munch() wrapper, but the output from serializable() isn't
    used for any purpose where Munch would be needed.
    
    Fixes: rhbz#1899274