Try to run tests for Windows and macOS.

.github/workflows/testing.yml

@@ -13,10 +13,11 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [3.6, 3.7, 3.8]
 
     steps:
@@ -25,6 +26,8 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+    - name: Display Python version
+        run: python -c "import sys; print(sys.version)"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
@@ -37,4 +40,4 @@ jobs:
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        pytest -vv
+        python -m pytest -vv