drop python 3.6 support

Author: mschrimpf

setup.py

@@ -34,7 +34,6 @@
         'Intended Audience :: Developers',
         'License :: OSI Approved :: MIT License',
         'Natural Language :: English',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
     ],
     test_suite='tests',