make pluggy an explicit test dependency

.github/workflows/python_package.yaml

@@ -29,7 +29,7 @@ jobs:
  - name: Install test dependencies
  run: |
  python -m pip install --upgrade pip
- pip install setuptools wheel
+ pip install setuptools wheel pluggy
  pip install flake8 pytest pytest-cov coveralls
  - name: Install package requirements
  run: pip install -r requirements.txt