yapf: support --parallel flag

see
https://github.com/google/yapf#usage

Bug: 1073708
Change-Id: I3824bbecc6c52d6b7f82aaa05e5108a8619124d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2162227Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>

yapf

@@ -5,6 +5,10 @@
 #   name: "infra/python/wheels/yapf-py2_py3"
 #   version: "version:0.24.0"
 # >
+# wheel: <
+#   name: "infra/python/wheels/futures-py2_py3"
+#   version: "version:3.1.1"
+# >
 # [VPYTHON:END]
 
 # -*- coding: utf-8 -*-