using pip to install pytest

.github/workflows/main.yml

@@ -36,7 +36,7 @@ jobs:
         uses: ./.github/actions/setup-poetry-env-only-main
 
       - name: just add pytest for running tests
-        run: poetry add pytest
+        run: poetry run python -m pip install -U pytest
 
       - name: Check if documentation can be built
         run: make test-only-main