[Security] Bump requests from 2.19.1 to 2.21.0

Bumps [requests](https://github.com/requests/requests) from 2.19.1 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@v2.19.1...v2.21.0)

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

requirements.txt

@@ -3,6 +3,6 @@ html5lib==1.0.1
 nose==1.3.7
 mock==2.0.0
 lxml==4.2.5
-requests==2.19.1
+requests==2.21.0
 BeautifulSoup4==4.6.3
 -e .