Revert "Update vpython spec for yapf"

This reverts commit 2b8a99b1.

Reason for revert: `git cl format` returning KeyError: 'chromium' in some chromium py repos

See https://crbug.com/1264076

Original change's description:
> 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>

Bug: 1264076
Change-Id: Ic94b6314f2c5183353d38f62d379304d7a830969
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3248264
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>

yapf

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