build(deps): update gevent requirement

Updates the requirements on [gevent](https://github.com/gevent/gevent) to permit the latest version.
- [Release notes](https://github.com/gevent/gevent/releases)
- [Changelog](https://github.com/gevent/gevent/blob/master/docs/changelog_pre.rst)
- [Commits](gevent/gevent@22.10.2...26.4.0)

---
updated-dependencies:
- dependency-name: gevent
  dependency-version: 26.4.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements/testing.txt

@@ -2,5 +2,5 @@
 pytest>=6.2.5,<9
 pytest-cov>=3,<8
 Flask>=2.0.3,<4 
-gevent>=22.10.2,<26
+gevent>=22.10.2,<27
 gevent-websocket>=0.10.1,<1