[FIX] install package itself

.github/workflows/python-package.yml

@@ -27,6 +27,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install flake8 pytest pytest-cov
+          python -m pip install -e .
           if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
       - name: Lint with flake8
         run: |