Update pytest command. (keon#831)

Invoke pytest as a python module rather than a standalone executable.

.github/workflows/python-app.yml

@@ -33,4 +33,4 @@ jobs:
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        pytest
+        python -m pytest