Remove unsupported trove classifier

Author: daneah

setup.cfg

@@ -21,7 +21,6 @@ classifiers =
     Programming Language :: Python
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9