update six dependency (#709)

table extensions use viewkeys

requirements.txt

@@ -1,4 +1,4 @@
-six >=1.6
+six >=1.9
 futures <=2.2.0
 # Futures is not required for Python 3, but it works up through 2.2.0 (after which it introduced breaking syntax).
 # This is left here to make sure install -r works with any runtime. When installing via setup.py, futures is omitted

setup.py

@@ -396,7 +396,7 @@ def run_setup(extensions):
         else:
             sys.stderr.write("Bypassing Cython setup requirement\n")
 
-    dependencies = ['six >=1.6']
+    dependencies = ['six >=1.9']
 
     if not PY3:
         dependencies.append('futures')