Avoid duplicate test runs in CI

Tox is running the tests for every Python instance listed in tox.ini
when CI should just be calling pytest.

.github/workflows/ci.yml

@@ -39,15 +39,12 @@ jobs:
         pip install dist/*.whl
       shell: bash
 
-    - name: Install test dependencies
-      run: pip install tox
-
     - name: Run unit tests
       run: |
         git config --global user.email "[email protected]"
         git config --global user.name "Your Name"
         cd wpiformat
-        tox
+        pytest
 
     - name: wpiformat - whole repo
       run: |