gh-35705: tox.ini / GH Actions: Remove gentoo-python3.9
### :books: Description
We remove one line from `tox.ini` and one line from `docker.yml`
Since there will be no fresh docker images for python3.9 on gentoo, GH
Actions will start to fail (sooner or later)
Resolves #35704
### :memo: Checklist
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- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35705
Reported by: Andrew
Reviewer(s): Matthias Köppe
Committer
Release Manager
