Update py-service_identity to 16.0.0.

16.0.0 (2016-02-18)
-------------------

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

- Python 3.3 and 2.6 aren't supported anymore.
  They may work by chance but any effort to keep them working has ceased.

  The last Python 2.6 release was on October 29, 2013 and isn't supported by the CPython core team anymore.
  Major Python packages like Django and Twisted dropped Python 2.6 a while ago already.

  Python 3.3 never had a significant user base and wasn't part of any distribution's LTS release.
- pyOpenSSL versions older than 0.14 are not tested anymore.
  They don't even build with recent OpenSSL versions.

Changes:
^^^^^^^^

- Officially support Python 3.5.
- ``service_identity.SubjectAltNameWarning`` is now raised if the server certicate lacks a proper ``SubjectAltName``.
  [`#9 <https://github.com/pyca/service_identity/issues/9>`_]
- Add a ``__str__`` method to ``VerificationError``.
- Port from ``characteristic`` to its spiritual successor `attrs <https://attrs.readthedocs.org/>`_.

(wiz)