added python 3.7 to classifier

setup.py

@@ -91,6 +91,7 @@ classes = """
     Programming Language :: Python :: 3.4
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy
     Operating System :: OS Independent