Fix Python classifiers

Author: avalentino

setup.cfg

@@ -35,7 +35,7 @@ classifiers =
     Operating System :: POSIX
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: Only
+    Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy
     Programming Language :: Cython