Add a summary job that checks whether all tests passed

We can use this to add a status check to ensure that only PRs passing
all tests can be merged.

The commit ID I used for the alls-green action is taken from the current
tip of the release/v1 branch. (It's not clear whether the action is
maintained, but using a commit hash means we don't have to care about
any future changes unless we actively decide to.)

Author: diazona

.github/workflows/tests.yml

@@ -62,3 +62,13 @@ jobs:
     - name: Test with tox with forced minimum dependencies
       if: matrix.versions.min-deps
       run: tox -vv -e py,py-smtp ${{ matrix.versions.min-deps }}
+
+  summary:
+    runs-on: ubuntu-latest
+    needs:
+    - build
+    steps:
+    - name: Summarize job statuses
+      uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe
+      with:
+        jobs: ${{ toJSON(needs) }}