add a requires-python

https://stackoverflow.com/questions/72373093/how-to-define-python-requires-in-pyproject-toml-using-setuptools

pyproject.toml

@@ -9,6 +9,7 @@ license = {file = "LICENSE"}
 classifiers = ["License :: OSI Approved :: MIT License"]
 version = "2023.6.27"
 readme = "README.rst"
+requires-python = ">=3.9"
 dependencies = [
     "archspec",
     "rich",