Bump gevent from 20.9.0 to 21.8.0

Bumps [gevent](https://github.com/gevent/gevent) from 20.9.0 to 21.8.0.
- [Release notes](https://github.com/gevent/gevent/releases)
- [Changelog](https://github.com/gevent/gevent/blob/master/docs/changelog_pre.rst)
- [Commits](https://github.com/gevent/gevent/compare/20.9.0...21.8.0)

---
updated-dependencies:
- dependency-name: gevent
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

requirements.txt

@@ -12,7 +12,7 @@ Flask-Login==0.5.0
 Flask-Migrate==2.5.3
 Flask-SQLAlchemy==2.4.4
 Flask-WTF==0.14.3
-gevent==20.9.0
+gevent==21.8.0
 greenlet==0.4.17
 idna==2.10
 itsdangerous==1.1.0