Lint the tests with flake8-pytest-style

This is a bit less strict overall than pylint, but enforces pytest idioms

.github/workflows/pytest.yml

@@ -17,8 +17,11 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install pytest
+        python -m pip install pytest flake8 flake8-pytest-style
         pip install .
+    - name: Lint the test files with flake8-pytest-style
+      run: |
+        flake8 tests/
     - name: Test with pytest
       run: |
         pytest