Bump eventlet from 0.19.0 to 0.31.0

Bumps [eventlet](https://github.com/eventlet/eventlet) from 0.19.0 to 0.31.0.
- [Release notes](https://github.com/eventlet/eventlet/releases)
- [Changelog](https://github.com/eventlet/eventlet/blob/master/NEWS)
- [Commits](https://github.com/eventlet/eventlet/compare/v0.19.0...v0.31.0)

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

requirements.txt

@@ -1,6 +1,6 @@
 Flask==0.12.4
 Flask-SocketIO==2.6.2
-eventlet==0.19.0
+eventlet==0.31.0
 greenlet==0.4.10
 python-dateutil==2.5.3
 python-engineio==0.9.2