Add Python 3.7 Trove classifier (python#6206)

The only thing we technically don't yet support is that in 3.7,
'async' and 'await' are always keywords.  But this is at best a false
negative.  Similarly, mypy doesn't change its behavior based on the
presence of 'from __future__ import annotations' -- it always
implements the 3.7 behavior for annotations.

setup.py

@@ -147,6 +147,7 @@ def run(self):
     'Programming Language :: Python :: 3.4',
     'Programming Language :: Python :: 3.5',
     'Programming Language :: Python :: 3.6',
+    'Programming Language :: Python :: 3.7',
     'Topic :: Software Development',
 ]