Update marshmallow to 3.7.1

[pyup-bot]

This PR updates marshmallow from 3.6.1 to 3.7.1.

Changelog

3.7.1

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

Bug fixes:

- ``fields.Boolean`` correctly serializes non-hashable types (:pr:`1633`).
Thanks :user:`jun0jang` for the PR.

3.7.0

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

Deprecations:

- `marshmallow.pprint` is deprecated and will be removed in marshmallow 4 (:issue:`1588`).

Support:

- Document ``default_error_messages`` on field classes (:pr:`1619`). Thanks :user:`weeix`.

Bug fixes:

- Fix passing ``only`` and ``exclude`` to ``Nested`` with an ordered ``Schema`` (:pr:`1627`).
Thanks :user:`juannorris` for the PR.

Links

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

[coveralls]

Pull Request Test Coverage Report for Build 784

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 781:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[pyup-bot]

Closing this in favor of #187