Update setup.cfg

* add 3.9 to badge

Author: Keith Roberts

setup.cfg

@@ -18,6 +18,7 @@ classifiers =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Topic :: Scientific/Engineering
     Topic :: Scientific/Engineering :: Mathematics
     Topic :: Scientific/Engineering :: Physics