Add parallel build to coveralls

It seems we need this when running a matrix of different python versions, as coveralls errors out when we send multiple reports for the same job.

.github/workflows/build.yaml

@@ -51,6 +51,10 @@ jobs:
         DB_PORT: 5432
     - name: Generate and send coveralls report
       uses: coverallsapp/github-action@v2
+      with:
+        parallel: true
+        flag-name: run-${{ matrix.python-version }}
+
 
   publish_to_pypi:
     # Only run this job if the run_tests job has succeeded, and if
@@ -103,3 +107,11 @@ jobs:
           pull: true
           push: true
           tags: ${{ env.DOCKER_REPOSITORY }}:${{ env.DOCKER_IMAGE_TAG }}
+
+finish:
+  runs-on: ubuntu-latest
+  steps:
+    - name: Close parallel build
+      uses: coverallsapp/github-action@v1
+      with:
+        parallel-finished: true