Update urllib3 to 1.26.4

[pyup-bot]

This PR updates urllib3 from 1.26.2 to 1.26.4.

Changelog

1.26.3

-------------------

* Fixed bytes and string comparison issue with headers (Pull 2141)

* Changed ``ProxySchemeUnknown`` error message to be
more actionable if the user supplies a proxy URL without
a scheme. (Pull 2107)

Links

• PyPI: https://pypi.org/project/urllib3
• Changelog: https://pyup.io/changelogs/urllib3/
• Docs: https://urllib3.readthedocs.io/

[pyup-bot]

Closing this in favor of #93