Introduce a gate/check GHA job

This adds a GHA job that reliably determines if all the required
dependencies have succeeded or not.

It also allows to reduce the list of required branch protection CI
statuses to just one — `check`. This reduces the maintenance burden
by a lot and have been battle-tested across a small bunch of projects
in its action form and in-house implementations of other people.

This action is now in use in aiohttp (and other aio-libs projects),
CherryPy, conda, coveragepy, Open edX, Towncrier some of the Ansible
repositories, pip-tools, all of the jaraco's projects (like
`setuptools`, `importlib_metadata`), some of hynek's projects (like
`attrs`, `structlog`), some PyCQA, PyCA, PyPA and pytest projects, a
few AWS Labs projects. Admittedly, I maintain a few of these but it
seems to address some of the pain folks have:
jaraco/skeleton#55 (comment).

I figured, this might be useful for MyST too, which is why I'm
submitting this patch.

The story behind this is explained in more detail at
https://github.com/marketplace/actions/alls-green#why.

.github/workflows/tests.yml

@@ -36,6 +36,12 @@ jobs:
           python-version: "3.8"
           sphinx: ">=4,<5"
 
+    continue-on-error: >-
+      ${{
+        (matrix.python-version == '3.8' && matrix.sphinx == '>=5,<6')
+        && true || false
+      }}
+
     runs-on: ${{ matrix.os }}
 
     steps:
@@ -103,10 +109,29 @@ jobs:
     - name: Run docutils CLI
       run: echo "test" | myst-docutils-html
 
+  # https://github.com/marketplace/actions/alls-green#why
+  check:  # This job does nothing and is only used for the branch protection
+
+    if: always()
+
+    needs:
+    - pre-commit
+    - tests
+    - check-myst-docutils
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}
+
   publish:
 
     name: Publish myst-parser to PyPi
-    needs: [pre-commit, tests, check-myst-docutils]
+    needs:
+    - check
     if: github.event_name == 'push' && startsWith(github.event.ref, 'refs/tags')
     runs-on: ubuntu-latest
     steps: