Install typing package on all environments.

Python-version-specific dependencies were only added to a recent version of setuptools; this approach is more compatible.

setup.py

@@ -39,15 +39,9 @@
     'filters',
     'requests',
     'six',
+    'typing',
   ],
 
-  # http://stackoverflow.com/a/33451105/
-  extras_require = {
-    ':python_version < "3.5"': [
-      'typing',
-    ],
-  },
-
   test_suite    = 'test',
   test_loader   = 'nose.loader:TestLoader',
   tests_require = [