Bump eventlet from 0.19.0 to 0.31.0 in /.circle

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](eventlet/eventlet@v0.19.0...v0.31.0)

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

Author: dependabot[bot]

.circle/requirements-pack-tests.txt

@@ -2,4 +2,4 @@ mock>=1.3.0,<2.1
 unittest2>=1.1.0,<2.0
 nose>=1.3.7
 # temporary workaround for travis issue
-eventlet==0.19.0
+eventlet==0.31.0