Remove the 3 :: Only classifier.

It doesn't do much, and who knows what the case
will be when Py4 is out.

Author: Julian

setup.cfg

@@ -15,7 +15,6 @@ classifiers =
     License :: OSI Approved :: MIT License
     Operating System :: OS Independent
     Programming Language :: Python
-    Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8