Add Coveralls parallel build coverage

Resolves #180

**Why this change was necessary**
We want to get an aggregate coverage report for each matrix build
option.

**What this change does**
 - Names coverage reports to indicate OS and Python version.
 - Runs a Coveralls finishing step to let Coveralls know the parallel
 build is finished.

**Any side-effects?**
None

**Additional context/notes/links**
https://coveralls-python.readthedocs.io/en/latest/usage/configuration.html#github-actions-gotcha

.github/workflows/tests.yml

@@ -41,9 +41,25 @@ jobs:
       - name: Run CI script
         run: ./ci.sh
 
-      - name: Coveralls
-        if: contains(matrix.os, 'ubuntu-18.04')
+      - name: Upload Coverage
+        run: coveralls
         env:
-          GITHUB_TOKEN: ${{ secrets.github_token }}
-        run: |
-          coveralls
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          COVERALLS_FLAG_NAME: ${{ matrix.os }}-Python-${{ matrix.python-version }}
+          COVERALLS_PARALLEL: true
+  coveralls:
+    name: Finish Coveralls
+    needs: test
+    runs-on: 'ubuntu-18.04'
+    steps:
+
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.7'
+    - name: Finished
+      run: |
+        pip3 install --upgrade coveralls
+        coveralls --finish
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}