Update python-decouple to 3.7

[pyup-bot]

This PR updates python-decouple from 3.3 to 3.7.

Changelog

3.7

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- Fix `Csv` cast hanging with `default=None`, now returning an empty list. (149)

3.6

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- Add support for Docker secrets.
- Fix deprecation warning on Python 3.10

3.5

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- Fix: fix syntax warnings due to comparison of literals using `is`
- Fix: avoid DeprecationError on ConfigParser.readfp()
- Add Tox Github Action
- Documentation fixups
- Security: bump Pygments version to >=2.7.4
- Fix .env -file quote stripping
- Changelog catchups for 3.2 and 3.3

3.4

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- Add choices helper
- Documentation fixups

Links

• PyPI: https://pypi.org/project/python-decouple
• Changelog: https://pyup.io/changelogs/python-decouple/
• Repo: http://github.com/henriquebastos/python-decouple/