Bump requests from 2.18.4 to 2.20.0 in /service

Bumps [requests](https://github.com/requests/requests) from 2.18.4 to 2.20.0.
- [Release notes](https://github.com/requests/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/master/HISTORY.md)
- [Commits](https://github.com/requests/requests/compare/v2.18.4...v2.20.0)

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

service/requirements.txt

@@ -14,6 +14,6 @@ pytest==3.4.0
 python-dateutil==2.6.1
 python-mimeparse==1.6.0
 pytz==2017.3
-requests==2.18.4
+requests==2.20.0
 six==1.11.0
 urllib3==1.22