Update marshmallow to 3.9.1

[pyup-bot]

This PR updates marshmallow from 3.9.0 to 3.9.1.

Changelog

3.9.1

******************

Bug fixes:

- Cast to mapping type in ``Mapping.serialize`` and ``Mapping.deserialize``
(:pr:`1685`).
- Fix bug letting ``Dict`` pass invalid dict on deserialization when no key or
value ``Field`` is specified (:pr:`1685`).

Links

• PyPI: https://pypi.org/project/marshmallow
• Changelog: https://pyup.io/changelogs/marshmallow/
• Repo: https://github.com/marshmallow-code/marshmallow

[pyup-bot]

Closing this in favor of #239