Stop using python setup.py test, Use PEP 517 workflow, Use SPDX license

• Replace python-pytest-runner with python-pytest
• Change to a PEP 517 based workflow
• Change to SPDX license identifier
  • https://github.com/mchaput/whoosh/blob/v2.7.4/LICENSE.txt

Part of upstream commit f5a777572fb9c91fa10be3c1ffd5f65cc9323653 is needed to pass check without pytest-runner while the commit also introduces a new test failure.