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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>