Configure GitHub workflow to publish coverage

Re-add coverage publishing on coveralls.io, formerly performed by
Travis CI (prior to theupdateframework#1242), using the coveralls cli tool according
to the documentation:
https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-gotcha

**Considered alternatives:**
- Official coveralls GitHub action, which does not seem to work
  well for Python:
  coverallsapp/github-action#4
  coverallsapp/github-action#30

- Inofficial fork of that action, which seems to work better
  but had issues finding the coverage data in the tests folder,
  or the covered code respectively.
  https://github.com/AndreMiras/coveralls-python-action

Besides aforementioned issues of these actions the use of cli tools
from curated package managers seems slightly preferable over
actions from the GitHub Marketplace (see theupdateframework#1246).

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.github/workflows/ci.yml

@@ -50,7 +50,37 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install tox
+          python -m pip install --upgrade tox coveralls
 
       - name: Run tox
         run: tox -e ${{ matrix.toxenv }}
+
+      - name: Publish on coveralls.io
+        env:
+          COVERALLS_FLAG_NAME: ${{ runner.os }} / Python ${{ matrix.python-version }}
+          COVERALLS_PARALLEL: true
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        # Use cp workaround to publish coverage reports with relative paths
+        # FIXME: Consider refactoring the tests to not require the test
+        # aggregation script being invoked from the `tests` directory, so
+        # that `.coverage` is written to and .coveragrc can also reside in
+        # the project root directory as is the convention.
+        run: |
+          cp tests/.coverage .
+          coveralls --rcfile=tests/.coveragerc
+
+  coveralls-fin:
+    # Always run when all 'build' jobs have finished even if they failed
+    # TODO: Replace always() with a 'at least one job succeeded' expression
+    # see https://docs.github.com/en/free-pro-team@latest/actions/reference/context-and-expression-syntax-for-github-actions
+    if: always()
+    needs: build
+    runs-on: ubuntu-latest
+    container: python:3-slim
+    steps:
+      - name: Finalize publishing on coveralls.io
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          pip3 install --upgrade coveralls
+          coveralls --finish