Update vpython spec for yapf to 0.31.0

R=sokcevic@google.com

Bug: 1306598
Change-Id: Idd5d4216dad62bf1c2d28d27e415046d6b1a311a
Follow-up-of: https://crrev/c/4284800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4289682
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Thiago Perrotta <tperrotta@chromium.org>

yapf

@@ -3,7 +3,7 @@
 # [VPYTHON:BEGIN]
 # wheel: <
 #   name: "infra/python/wheels/yapf-py2_py3"
-#   version: "version:0.29.1"
+#   version: "version:0.31.0"
 # >
 # wheel: <
 #   name: "infra/python/wheels/futures-py2_py3"