Add Python 3.7 trove classifier (certifi#94)

Python 3.7 was released on June 27, 2018.

https://docs.python.org/3/whatsnew/3.7.html

Author: jdufresne

setup.py

@@ -63,5 +63,6 @@
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
     ],
 )