👷 Fix Coveralls parallel reporting

Handle aggregated Coveralls reports using the `COVERALLS_PARALLEL`
environment variable and the `--finish` flag. This prevents duplicate
report submissions in the workflow matrix, which sometimes led to
422 "Unprocessable Entity" errors during upload:
```
Error running coveralls: Could not submit coverage: 422 Client Error:
Unprocessable Entity for url: https://coveralls.io/api/v1/jobs
```

Also updated the service name to `github-actions`, as suggested in:
- lemurheavy/coveralls-public#1607 (comment)
- https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-support

.github/workflows/test_python.yml

@@ -32,10 +32,24 @@ jobs:
       run: tox
     - name: Coveralls
       env:
-        COVERALLS_REPO_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        COVERALLS_SERVICE_NAME: github
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        COVERALLS_SERVICE_NAME: github-actions
+        COVERALLS_FLAG_NAME: python-${{ matrix.python }}-${{ matrix.os }}
+        COVERALLS_PARALLEL: true
       run: coveralls
 
+  Coveralls:
+    needs: Tests
+    runs-on: ubuntu-latest
+    container: python:3-slim
+    steps:
+    - name: Finished
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        pip install coveralls
+        coveralls --finish
+
   Docker:
     runs-on: ubuntu-latest
     steps:
@@ -55,4 +69,3 @@ jobs:
       run: sphinx-build -b linkcheck docs/source docs/build
     - name: Generate documentation
       run: sphinx-build docs/source docs/build
-