Bump eventlet from 0.30.2 to 0.31.0 in /checker/src

Bumps [eventlet](https://github.com/eventlet/eventlet) from 0.30.2 to 0.31.0.
- [Release notes](https://github.com/eventlet/eventlet/releases)
- [Changelog](https://github.com/eventlet/eventlet/blob/master/NEWS)
- [Commits](eventlet/eventlet@v0.30.2...v0.31.0)

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

checker/src/requirements.txt

@@ -5,7 +5,7 @@ dnspython==1.16.0
 enochecker==0.3.2
 enochecker-cli==0.6.0
 enochecker-core==0.8.1
-eventlet==0.30.2
+eventlet==0.31.0
 Flask==1.1.2
 greenlet==1.0.0
 gunicorn==20.1.0