[gh/actions] Add full test matrix

Author: mpsonntag

.github/workflows/run-tests.yml

@@ -15,8 +15,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
-        python-version: [3.6]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}