Bump gevent from 21.8.0 to 21.12.0

Bumps [gevent](https://github.com/gevent/gevent) from 21.8.0 to 21.12.0.
- [Release notes](https://github.com/gevent/gevent/releases)
- [Changelog](https://github.com/gevent/gevent/blob/master/docs/changelog_pre.rst)
- [Commits](gevent/gevent@21.8.0...21.12.0)

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

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

requirements.txt

@@ -18,4 +18,4 @@ cryptography==36.0.1
 alembic==1.7.4
 PyJWT==2.3.0
 Jinja2==3.0.3
-gevent==21.8.0
+gevent==21.12.0