Add Python 3.6/3.7 trove classifiers to document support

Python 3.6 support was added in
7a1f48e.

Python 3.7 testing was added in
ae05bd9.

Document general Python 2 support as well.

Author: jdufresne

setup.py

@@ -101,6 +101,8 @@
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.4",
     "Programming Language :: Python :: 3.5",
+    "Programming Language :: Python :: 3.6",
+    "Programming Language :: Python :: 3.7",
     "License :: OSI Approved",
     "License :: OSI Approved :: MIT License",
     "Development Status :: 4 - Beta",