make pip wheel building CI test work again

As pytest and pytest-xdist are no longer in requirements.txt, we need to install these manually in this test script.

.github/workflows/pull-request.yml

@@ -156,6 +156,7 @@ jobs:
       - name: "Run Python Tests"
         run: |
           source .venv/bin/activate
+          pip install pytest pytest-xdist
           cd src && pytest -n auto -m "not ciSkip"