Bump gevent from 1.4.0 to 20.6.2

Bumps [gevent](https://github.com/gevent/gevent) from 1.4.0 to 20.6.2.
- [Release notes](https://github.com/gevent/gevent/releases)
- [Changelog](https://github.com/gevent/gevent/blob/master/docs/changelog_1_4.rst)
- [Commits](gevent/gevent@1.4.0...20.6.2)

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

requirements.txt

@@ -2,5 +2,5 @@ Jinja2==2.10.3
 Werkzeug==0.16.0
 click==7.0
 gevent-websocket==0.10.1
-gevent==1.4.0
+gevent==20.6.2
 watchdog==0.9.0