www/py-django-prometheus: Convert to PEP517

• Switch to the PEP517 build framework and remove py-pytest-runner as a build time dependency while I'm here, because upstream of it has archived the project with the following notice:
  
  pytest-runner depends on deprecated features of setuptools and relies on features that break security mechanisms in pip. For example 'setup_requires' and 'tests_require' bypass pip --require-hashes. See also https://github.com/pypa/setuptools/issues/1684

• Bump PORTREVISION due package change.