Bump requests from 2.7.0 to 2.20.0

Bumps [requests](https://github.com/requests/requests) from 2.7.0 to 2.20.0.
- [Release notes](https://github.com/requests/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/master/HISTORY.md)
- [Commits](psf/requests@v2.7.0...v2.20.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -2,5 +2,5 @@ futures==3.0.3
 gevent==1.0.2
 greenlet==0.4.7
 grequests==0.2.0
-requests==2.7.0
+requests==2.20.0
 Sphinx==1.3.1