Bump eventlet from 0.20.1 to 0.31.0 in /examples/server/wsgi

Bumps [eventlet](https://github.com/eventlet/eventlet) from 0.20.1 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/v0.20.1...v0.31.0)

Signed-off-by: dependabot[bot] <[email protected]>

examples/server/wsgi/requirements.txt

@@ -1,7 +1,7 @@
 Click==7.0
 enum-compat==0.0.2
 enum34==1.1.6
-eventlet==0.20.1
+eventlet==0.31.0
 Flask==1.0.2
 greenlet==0.4.12
 itsdangerous==1.1.0