build(deps-dev): bump requests from 2.19.1 to 2.20.0

Bumps [requests](https://github.com/requests/requests) from 2.19.1 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@v2.19.1...v2.20.0)

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

requirements-dev.txt

@@ -21,7 +21,7 @@ pyparsing==2.2.0
 pytest-cov==2.5.1
 pytest==3.6.1
 pytz==2018.4
-requests==2.19.1
+requests==2.20.0
 six==1.11.0
 snowballstemmer==1.2.1
 sphinx==1.7.5