Update version of six used in .vpython3 to match .vpython.

Bug: chromium:1007101
Change-Id: Ic6ced991520288f18638ecf780a5ecf87781b6a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596045Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838480}

.vpython3

@@ -28,5 +28,5 @@ python_version: "3.8"
 # become available.
 wheel: <
   name: "infra/python/wheels/six-py2_py3"
-  version: "version:1.10.0"
+  version: "version:1.15.0"
 >