fix six package path.

--strip_prefix doesn't seem to work.
diff --git a/six.BUILD b/six.BUILD
index a259232..55c53a4 100644
--- a/six.BUILD
+++ b/six.BUILD
@@ -1,5 +1,12 @@
+genrule(
+  name = "copy_six",
+  srcs = ["six-1.10.0/six.py"],
+  outs = ["six.py"],
+  cmd = "cp $< $(@)",
+)
+
 py_library(
   name = "six",
-  srcs = ["six-1.10.0/six.py"],
+  srcs = ["six.py"],
   visibility = ["//visibility:public"],
 )