Update python-service_identity to version 21.1.0 / rev 10 via SR 952143
    
    https://build.opensuse.org/request/show/952143
    by user mcepl + dimstar_suse
    - Update to 21.1.0:
      - Python 3.4 is not supported anymore. It has been unsupported
        by the Python core team for a while now, its PyPI downloads
        are negligible, and our CI provider removed it as a supported
        option.
      - It's very unlikely that service-identity will break under 3.4
        anytime soon, which is why we do not block its installation
        on Python 3.4. But we don't test it anymore and will block it
        once someone reports breakage.
      - service_identity.exceptions.VerificationError can now be
        pickled and is overall more well-behaved as an exception.
        This raises the requirement of attrs to 19.1.0.
    - This package actually truly requires six (it is expected to
      work with Python 2.7 as well).