Update list of supported Python versions

The goal of these changes is to provide actual
information about supported Python versions.
I believe that only versions for those you have
tests on CI should be listed as supported.

setup.py

@@ -53,13 +53,12 @@ setup(
         'License :: OSI Approved :: MIT License',
         'Natural Language :: English',
         "Programming Language :: Python :: 2",
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.3',
         'Programming Language :: Python :: 3.4',
         'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
+        'Programming Language :: Python :: 3.7',
     ],
     test_suite='tests',
     tests_require=test_requirements