Drop Python 3.6 and 3.7 from classifiers

Author: frenzymadness

setup.cfg

@@ -13,8 +13,6 @@ license = BSD-3-Clause
 license_files = LICENSE.txt
 classifiers =
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10