Add backend-path for future Pips

Co-Authored-By: Paul Ganssle <paul@ganssle.io>

pyproject.toml

 [build-system]
 requires = ["setuptools >= 40.8", "wheel"]
 build-backend = "setuptools.build_meta"
+backend-path = ["."]
 
 [tool.towncrier]
     package = "setuptools"