make sure the right pytest is used

.github/workflows/unit_tests.yml

@@ -29,9 +29,9 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install pytest
-          if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+          pip install -r requirements.txt
 
       - name: Test with pytest
         run: |
           cd tests
-          pytest --pyargs .
+          python -m pytest --pyargs .