Update requests to 2.21.0

[pyup-bot]

This PR updates requests from 2.19.1 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).

2.20.0

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

**Bugfixes**

-   Content-Type header parsing is now case-insensitive (e.g.
 charset=utf8 v Charset=utf8).
-   Fixed exception leak where certain redirect urls would raise
 uncaught urllib3 exceptions.
-   Requests removes Authorization header from requests redirected
 from https to http on the same hostname. (CVE-2018-18074)
-   `should_bypass_proxies` now handles URIs without hostnames (e.g.
 files).

**Dependencies**

- Requests now supports urllib3 v1.24.

**Deprecations**

- Requests has officially stopped support for Python 2.6.

Links

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

[pyup-bot]

Closing this in favor of #962