[Security] Bump requests from 2.18.4 to 2.21.0

Bumps [requests](https://github.com/requests/requests) from 2.18.4 to 2.21.0. **This update includes security fixes.**
- [Release notes](https://github.com/requests/requests/releases)
- [Changelog](https://github.com/requests/requests/blob/master/HISTORY.md)
- [Commits](psf/requests@v2.18.4...v2.21.0)

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

requirements.txt

@@ -62,7 +62,7 @@ python3-openid==3.1.0
 pytz==2017.2
 PyYAML==3.12
 redis==2.10.6
-requests==2.18.4
+requests==2.21.0
 requests-oauthlib==0.8.0
 six==1.11.0
 sqlparse==0.2.4