Bump gevent from 1.5.0 to 20.4.0

Bumps [gevent](https://github.com/gevent/gevent) from 1.5.0 to 20.4.0.
- [Release notes](https://github.com/gevent/gevent/releases)
- [Changelog](https://github.com/gevent/gevent/blob/master/docs/changelog_1_5.rst)
- [Commits](gevent/gevent@1.5.0...20.04.0)

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

Author: dependabot-preview[bot]

poetry.lock

@@ -11,7 +11,7 @@ marshmallow = "^3.4.0"
 webargs = "^6.0.0"
 apispec = "^3.2.0"
 flask-cors = "^3.0.8"
-gevent = "^1.4.0"
+gevent = ">=1.4,<21.0"
 gevent-websocket = "^0.10.1"
 zeroconf = ">=0.24.5,<0.26.0"