Update bravado-core to 5.17.1

[pyup-bot]

This PR updates bravado-core from 5.16.0 to 5.17.1.

Changelog

5.17.0

-------------------
- Model instances are now pickleable - `PR 372`_
- Improve equality checks, most notably fixing an issue when ``internally_dereference_refs`` was set to ``True`` - `PR 371`_

5.16.1

-------------------
.. warning::
 This is release is technically backward incompatible with 5.16.0 as it is removing the equality feature on Spec objects.
 We're **not** releasing a major version mostly because we don't expect users are using it.

 If you're a user using it please make sure to use the ``is_equal`` methods. They behave as the ``__eq__`` methods added in 5.16.0 without altering the hashability feature of the objects.

 Context can be found on `PR 363`_ and `PR 364`_.

- Replace __eq__ with is_equal to allow hashability - `PR 364`_
- Drop support for Python 3.5.0 and previous Python 3 versions - `PR 366`_
- Fix validation of nullable polymorphic objects - `Issue 359`_, `PR 367`_
- Update ``msgpack`` depedency (as ``msgpack-python`` is deprecated) - `PR 368`_ - Thanks vanschelven for your contribution.
- Improve typing annotations - `PR 361`_

Links

• PyPI: https://pypi.org/project/bravado-core
• Changelog: https://pyup.io/changelogs/bravado-core/
• Repo: https://github.com/Yelp/bravado-core