Bump gevent from 21.1.2 to 21.8.0 (#387)

Bumps [gevent](https://github.com/gevent/gevent) from 21.1.2 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/21.1.2...21.8.0

)

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

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