Update vpython spec for yapf

Update yapf to latest version.

Bug: None
Change-Id: I77a994105e8ef16dcaf61402523d4cf9a1e39722
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3237590
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Auto-Submit: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>

yapf

@@ -3,7 +3,7 @@
 # [VPYTHON:BEGIN]
 # wheel: <
 #   name: "infra/python/wheels/yapf-py2_py3"
-#   version: "version:0.27.0"
+#   version: "version:0.31.0"
 # >
 # wheel: <
 #   name: "infra/python/wheels/futures-py2_py3"