Updated py-requests-toolbelt to 0.8.0.

0.8.0 -- 2017-05-20
-------------------

More information about this release can be found on the `0.8.0 milestone`_.

New Features
~~~~~~~~~~~~

- Add ``UserAgentBuilder`` to provide more control over generated User-Agent
  strings.

Fixed Bugs
~~~~~~~~~~

- Include ``_validate_certificate`` in the lits of picked attributes on the
  ``AppEngineAdapter``.
- Fix backwards incompatibility in ``get_encodings_from_content``

.. _0.8.0 milestone:
    https://github.com/sigmavirus24/requests-toolbelt/milestones/0.8.0

(wiz)