Log Message:
py-service_identity: update to 21.1.0.

21.1.0 (2021-05-09)
-------------------

Backward-incompatible changes:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- 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.

Changes:
^^^^^^^^

- ``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.