Try fixing GitHub Actions to use the correct Python

Author: mgorny

.github/workflows/ci.yml

@@ -13,7 +13,9 @@ jobs:
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: ${{ matrix.python_version }}
+          python-version: ${{ matrix.python-version }}
+      - name: wtf
+        run: python --version
       - name: Install dependencies
         run: python -m pip install tox
       - name: Run tests