Update six library version

https://crrev.com/c/2837514 uses functionality that is not in the onld version. We need to update it to unblock chromium -> webrtc roll

Bug: None
Change-Id: Ie3052ff58606b2955aa3f984096f6ce20d3ad7b0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216241
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Andrey Logvin <landrey@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33835}
diff --git a/.vpython b/.vpython
index 92c9c51..df838dc 100644
--- a/.vpython
+++ b/.vpython
@@ -52,7 +52,7 @@
 
 wheel: <
   name: "infra/python/wheels/six-py2_py3"
-  version: "version:1.10.0"
+  version: "version:1.15.0"
 >
 wheel: <
   name: "infra/python/wheels/pbr-py2_py3"