Bump gevent from 21.8.0 to 21.12.0 (#459)

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](https://github.com/gevent/gevent/compare/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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

 cachetools==4.2.4
 diskcache==5.2.1
 Flask==2.0.2
-gevent==21.8.0
+gevent==21.12.0
 gunicorn==20.1.0
 fsspec==2021.10.0
 gcsfs==2021.10.0