add Python 3.9 to setup

Author: jGaboardi

setup.py

@@ -90,6 +90,7 @@ def setup_package():
             "Programming Language :: Python :: 3.6",
             "Programming Language :: Python :: 3.7",
             "Programming Language :: Python :: 3.8",
+            "Programming Language :: Python :: 3.9",
         ],
         package_data={"libpysal": list(example_data_files)},
         install_requires=install_reqs,