gh-35705: tox.ini / GH Actions: Remove gentoo-python3.9

### 📚 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

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->

- [x] The title is concise, informative, and self-explanatory.
- [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.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->

URL: #35705
Reported by: Andrew
Reviewer(s): Matthias Köppe

Author: Release Manager

.github/workflows/docker.yml

@@ -50,7 +50,6 @@ on:
            "centos-7-devtoolset-gcc_11",
            "centos-stream-8-python3.9",
            "centos-stream-9-python3.9",
-           "gentoo-python3.9",
            "gentoo-python3.10",
            "gentoo-python3.11",
            "archlinux-latest",

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=3420eff6767e23f15913f1293a1686a66d816453
-md5=56ba700a555b960eec313fa20261433e
-cksum=3706935509
+sha1=828899494028e64e114575d588ddb389ab8d3934
+md5=cea4edd6bca8a8ad4b654919dfa90ad1
+cksum=1753584513

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-21e8f9faf13b5d278c4289a7f818c8aa43d47c19
+9585a189b3d64daf382d733eb625f221509d731b

tox.ini

@@ -312,7 +312,6 @@ setenv =
     #
     gentoo:      SYSTEM=gentoo
     gentoo:      BASE_IMAGE=sheerluck/sage-on-gentoo-stage4
-    gentoo-python3.9: BASE_TAG=latest-py39
     gentoo-python3.10: BASE_TAG=latest-py10
     gentoo-python3.11: BASE_TAG=latest-py11
     gentoo:                                    IGNORE_MISSING_SYSTEM_PACKAGES=no