Loosen restriction on the version of six required. Fixes #947.

CHANGES.rst

+v34.0.3
+-------
+
+* #947: Loosen restriction on the version of six required,
+  restoring compatibility with environments relying on
+  six 1.6.0 and later.
+
 v34.0.2
 -------
 

setup.py

@@ -164,7 +164,7 @@ setup_params = dict(
     python_requires='>=2.6,!=3.0.*,!=3.1.*,!=3.2.*',
     install_requires=[
         'packaging>=16.8',
-        'six>=1.10.0',
+        'six>=1.6.0',
         'appdirs>=1.4.0',
     ],
     extras_require={