setup: Add Python 3.7 classifier

Author: 1st1

setup.py

@@ -252,6 +252,7 @@ def build_extensions(self):
         'Programming Language :: Python :: 3 :: Only',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'License :: OSI Approved :: Apache Software License',
         'License :: OSI Approved :: MIT License',
         'Intended Audience :: Developers',