Update requests to 2.21.0

[pyup-bot]

This PR updates requests from 2.20.1 to 2.21.0.

Changelog

2.21.0

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

**Dependencies**

- Requests now supports idna v2.8.

Links

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

[codecov]

Codecov Report

Merging #112 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #112   +/-   ##
=======================================
  Coverage   67.29%   67.29%           
=======================================
  Files          51       51           
  Lines        2162     2162           
=======================================
  Hits         1455     1455           
  Misses        707      707

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