Mark uvloop as Python 3-only

Author: elprans

setup.py

@@ -112,6 +112,7 @@ def build_extensions(self):
     ],
     classifiers=[
         'Development Status :: 4 - Beta',
+        'Programming Language :: Python :: 3 :: Only',
         'Programming Language :: Python :: 3.5',
         'License :: OSI Approved :: Apache Software License',
         'License :: OSI Approved :: MIT License',