Bump requests from 2.18.4 to 2.20.0 in /manager

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](psf/requests@v2.18.4...v2.20.0)

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

Author: dependabot[bot]

manager/requirements.txt

@@ -21,7 +21,7 @@ Pillow==8.3.2
 pommerman==0.2.0
 pyglet==1.4.0a1
 pytz==2018.4
-requests==2.18.4
+requests==2.20.0
 ruamel.yaml==0.15.37
 scipy==1.1.0
 six==1.11.0