Bump eventlet from 0.21.0 to 0.31.0

Bumps [eventlet](https://github.com/eventlet/eventlet) from 0.21.0 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/v0.21.0...v0.31.0)

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

requirements.txt

@@ -3,7 +3,7 @@ chardet==3.0.4
 click==6.7
 enum-compat==0.0.2
 enum34==1.1.6
-eventlet==0.21.0
+eventlet==0.31.0
 Flask==0.12.2
 greenlet==0.4.12
 idna==2.5