Update python version in setup.py (#630)

Author: altheaden

setup.py

@@ -27,7 +27,7 @@ def package_files(directory, prefixes, extensions):
     author="Ryan Forsyth, Chris Golaz",
     author_email="[email protected], [email protected]",
     description="Post-processing software for E3SM",
-    python_requires=">=3.6",
+    python_requires=">=3.9",
     packages=find_packages(include=["zppy", "zppy.*"]),
     package_data={"": data_files},
     entry_points={"console_scripts": ["zppy=zppy.__main__:main"]},