Update python requirement to 3.9

setup.py

@@ -229,7 +229,7 @@ def run(self):
     long_description_content_type="text/markdown",
     url="http://dedalus-project.org",
     classifiers=["Programming Language :: Python :: 3"],
-    python_requires=">=3.8",
+    python_requires=">=3.9",
     install_requires=install_requires,
     license="GPL3",
     packages=setuptools.find_packages(),