Update marshmallow to 3.4.0

[pyup-bot]

This PR updates marshmallow from 3.3.0 to 3.4.0.

Changelog

3.4.0

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

Features:

- Improve type coverage (:issue:`1479`). Thanks :user:`Reskov`.

Bug fixes:

- Fix typing for ``data`` param of ``Schema.load`` and ``ValidationError`` (:issue:`1492`).
Thanks :user:`mehdigmira` for reporting and thanks :user:`dfirst` for the PR.

Other changes:

- Remove unnecessary typecasts (:pr:`1500`). Thanks :user:`hukkinj1`. 
- Remove useless ``_serialize`` override in ``UUID`` field (:pr:`1489`).

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 544

• 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 71.739%

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Totals
Change from base Build 535:	0.0%
Covered Lines:	99
Relevant Lines:	138

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💛 - Coveralls