add supported python versions (#64)

Author: Maeda Tomoki

setup.py

@@ -28,6 +28,8 @@ def version():
                      'License :: OSI Approved :: MIT License',
                      'Programming Language :: Python :: 2.7',
                      'Programming Language :: Python :: 3',
+                     'Programming Language :: Python :: 3.6',
+                     'Programming Language :: Python :: 3.7',
                      'Topic :: Software Development :: Quality Assurance'],
         keywords='clean,fix,automatic,unused,import',
         py_modules=['autoflake'],