Always run yapf with vpython3.

The version we are using now does not run under python 2.7.

Add an explicit python_version 3.8 to the inline spec so that we don't
rely on vpython's default version. This is a no-op change when running with vpython3. We should run yapf under 3.11, but this needs additional testing and likely an update of zipp, so we'll do it separately.

Change-Id: If1fccbe6b13c77a770bd1a8c9de0789d543ba890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5198109
Auto-Submit: Brian Ryner <bryner@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

git_cl.py

@@ -6286,13 +6286,7 @@ def _RunYapf(opts, paths, top_dir, upstream_commit):
         if not yapf_style:
             yapf_style = 'pep8'
 
-        with open(path, 'r') as py_f:
-            if 'python2' in py_f.readline():
-                vpython_script = 'vpython'
-            else:
-                vpython_script = 'vpython3'
-
-        cmd = [vpython_script, yapf_tool, '--style', yapf_style, path]
+        cmd = ['vpython3', yapf_tool, '--style', yapf_style, path]
 
         if not opts.full:
             ranges = line_diffs.get(path)

yapf

@@ -1,6 +1,7 @@
 #!/usr/bin/env vpython3
 
 # [VPYTHON:BEGIN]
+# python_version: "3.8"
 # wheel: <
 #   name: "infra/python/wheels/yapf-py3"
 #   version: "version:0.40.2"