add pyproject.toml

see also https://packaging.python.org/en/latest/guides/modernize-setup-py-project/ and
pypa/pip#6334

Author: erwincoumans

pyproject.toml

@@ -0,0 +1,5 @@
+# pyproject.toml
+[build-system]
+requires = ["setuptools >= 42.0.0"]
+build-backend = "setuptools.build_meta"
+