Bump requests from 2.9.1 to 2.20.0

Bumps [requests](https://github.com/requests/requests) from 2.9.1 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@v2.9.1...v2.20.0)

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

requirements.txt

@@ -34,7 +34,7 @@ Pygments==1.6
 pytz==2015.7
 PyYAML==3.11
 raven==5.10.2
-requests==2.9.1
+requests==2.20.0
 requests-oauthlib==0.6.0
 simple-pbkdf2==1.0
 six==1.7.3