Update external build metadata to state explicit support for Python 3.7.

The OSS issue is here: #8

PiperOrigin-RevId: 237494949
Change-Id: I7f0b17164baf6f51007ad523d44e0a25e10e8e89

Author: Hamid Asaadi

src/setup.py

@@ -120,6 +120,7 @@ def ReadConfig(section, value, default):
     classifiers=[
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
         'Development Status :: 3 - Alpha',
         'Intended Audience :: Developers',
     ])