change classifier to allow twine upload

Author: rasbt

setup.cfg

@@ -7,7 +7,7 @@ classifiers =
     Intended Audience :: Developers
     Intended Audience :: Science/Research
     License :: OSI Approved :: BSD License
-    Programming Language :: Python :: 3 only
+    Programming Language :: Python :: 3
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9