Update stable branch after publishing to PyPI (#3223)

We've decided to a) convert stable back into a branch and b) to update
it immediately as part of the release process. We may as well automate
it. And about going back to a branch ...

Git tags are not the right tool, at all[^1]. They come with the
expectation that they will never change. Things will not work as
expected if they do change, doubly so if they change regularly. Once
you pull stable from the remote and it's copied in your local
repository, no matter how many times you run git pull you'll never see
it get updated automatically. Your only recourse is to delete the tag
via `git tag -d stable` before pulling.

This gets annoying really quickly since stable is supposed to be the
solution for folks "who want to move along as Black developers deem
the newest version reliable."[^2] See this comment for how this impacts
users using our Vim plugin[^3]. It also affects us developers[^4]. If
you have stable locally, once we cut a new release and update the stable
tag, a simple `git pull` / `git fetch` will not pull down the updated
stable tag. Unless you remember to delete stable before pulling, stable
will become stale and useless.

You can argue this is a good thing ("people should explicitly opt into
updating stable"), but IMO it does not match user expectations nor
developer expectations[^5]. Especially since not all our integrations
that use stable are bound by this security measure, for example our
GitHub Action (since it does a clean fetch of the repository every time
it's used). I believe consistency would be good here.

Finally, ever since we switched to a tag, we've been facing issues with
ReadTheDocs not picking up updates to stable unless we force a rebuild.
The initial rebuild on the stable update just pulls the commit the tag
previously pointed to. I'm not sure if switching back to a branch will
fix this, but I'd wager it will.

[^1]: https://git-scm.com/docs/git-tag#_on_re_tagging

[^2]: https://black.readthedocs.io/en/stable/contributing/release_process.html#moving-the-stable-tag

[^3]: #2503 (comment)

[^4]: In fairness, most folks working on Black probably don't use the
      `stable` ref anyway, especially us maintainers who'd know what is
      the latest version by heart, but it'd still be nice to make it
      usable for local dev though.

[^5]: Also what benefit does a `stable` ref have over explicit version
      tags like `22.6.0`? If you're going to opt into some odd pin
      mechanism, might as well use explicit version tags for clarity
      and consistency.

.github/workflows/pypi_upload.yml

@@ -74,3 +74,22 @@ jobs:
         env:
           TWINE_PASSWORD: ${{ secrets.PYPI_TOKEN }}
         run: pipx run twine upload --verbose -u '__token__' wheelhouse/*.whl
+
+  update-stable-branch:
+    name: Update stable branch
+    needs: [main, mypyc]
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - name: Checkout stable branch
+        uses: actions/checkout@v3
+        with:
+          ref: stable
+          fetch-depth: 0
+
+      - name: Update stable branch to release tag & push
+        run: |
+          git reset --hard ${{ github.event.release.tag_name }}
+          git push