Use typing_extensions

.github/workflows/python-tests.yml

@@ -25,12 +25,15 @@ jobs:
       - run: mypy .
   test:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.6, 3.7, 3.8]
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python
+      - name: Setup Python${{ matrix.python-version }}
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: ${{ matrix.python-version }}
           architecture: x64
       - name: Install dependencies
         run: pip install --progress-bar off .