Continuous integration: re-enable coveralls test coverage upload (#637)

* Restore Coveralls GitHub Action for Python

* Only attempt to run coveralls uploads on Linux platforms

* Add the coveralls repo token back in github actions

Co-authored-by: Hristo Harsev <hhursev@gmail.com>

.github/workflows/unittests.yaml

@@ -43,3 +43,10 @@ jobs:
       - name: Unittest and Coverage Report
         run: |
           python run_tests.py
+      # Provide code coverage reports on Linux
+      - if: ${{ matrix.os == 'ubuntu-latest' }}
+        name: coveralls.io
+        uses: AndreMiras/coveralls-python-action@develop
+        with:
+          # coveralls repo token
+          github-token: "SmlfzlVJy4ow55rduU7IU5GmmFCfAdGeq"