Update peewee to 3.9.1

[pyup-bot]

This PR updates peewee from 3.9.0 to 3.9.1.

Changelog

3.9.1

Includes a bugfix for an `AttributeError` that occurs when using MySQL with the
`MySQLdb` client.

[View commits](https://github.com/coleifer/peewee/compare/3.9.0...3.9.1)

Links

• PyPI: https://pypi.org/project/peewee
• Changelog: https://pyup.io/changelogs/peewee/
• Repo: https://github.com/coleifer/peewee/

[fkztw]

Oh, should've squashed commits before merge.

[coveralls]

Coverage remained the same at 84.15% when pulling 43a2735 on pyup-update-peewee-3.9.0-to-3.9.1 into 9d20bee on master.