Update requests to 2.21.0

[pyup-bot]

This PR updates requests from 2.20.0 to 2.21.0.

Changelog

2.21.0

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

**Dependencies**

- Requests now supports idna v2.8.

2.20.1

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

**Bugfixes**

- Fixed bug with unintended Authorization header stripping for
redirects using default ports (http/80, https/443).

Links

• PyPI: https://pypi.org/project/requests
• Changelog: https://pyup.io/changelogs/requests/
• Homepage: http://python-requests.org

[coveralls]

Coverage remained the same at 97.087% when pulling 283746b on pyup-update-requests-2.20.0-to-2.21.0 into 129144c on master.