Update pygithub requirement from ~=2.8 to ~=2.9

Updates the requirements on [pygithub](https://github.com/pygithub/pygithub) to permit the latest version.
- [Release notes](https://github.com/pygithub/pygithub/releases)
- [Changelog](https://github.com/PyGithub/PyGithub/blob/main/doc/changes.rst)
- [Commits](PyGithub/PyGithub@v2.8.0...v2.9.0)

---
updated-dependencies:
- dependency-name: pygithub
  dependency-version: 2.9.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements_maint.txt

@@ -1,3 +1,3 @@
-pygithub~=2.8
+pygithub~=2.9
 ruamel-yaml~=0.19
 Unidecode~=1.3