test: stabilize test workflow by removing Poetry lockfile coupling (#1258)

.github/workflows/test.yml

@@ -31,9 +31,7 @@ jobs:
           python-version: ${{ matrix.python }}
       - name: Setup Graphviz
         uses: ts-graphviz/setup-graphviz@v2
-      - name: Install poetry
-        run: curl -sSL https://install.python-poetry.org | python3 - --version 2.2.1
       - name: Run all tests
         run: |
-          poetry install
+          python -m pip install .
-          poetry run python -m unittest -v tests/*.py
+          python -m unittest -v tests/*.py