Use Coveralls Github Action to send data to coveralls again

This was lost when Travis stopped working, so our coveralls data
is frozen in 2021. This should get it working again using
https://github.com/marketplace/actions/coveralls-github-action


and allow coveralls to post comments on PRs.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

.github/workflows/tox.yml

 ---
 name: Test Python package with Tox
 
-on: [pull_request]
+on: ["push", "pull_request"]
 
 jobs:
   build:
@@ -34,6 +34,12 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install tox tox-gh-actions
+          pip install tox tox-gh-actions coverage
       - name: Test with tox
         run: tox
+      - name: Produce coverage.lcov for coveralls
+        if: matrix.python-version == '3.11'
+        run: coverage lcov
+      - name: Coveralls
+        if: matrix.python-version == '3.11'
+        uses: coverallsapp/github-action@v2