Use vpython3 when calling clang-format-diff.py

R=gavinmak@google.com

Change-Id: I4bee5d9e4d5857d359d9f0663a4fbeea1194f838
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3502623
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

git_cl.py

@@ -5077,7 +5077,7 @@ def _RunClangFormatDiff(opts, clang_diff_files, top_dir, upstream_commit):
     except clang_format.NotFoundError as e:
       DieWithError(e)
 
-    cmd = ['vpython', script, '-p0']
+    cmd = ['vpython3', script, '-p0']
     if not opts.dry_run and not opts.diff:
       cmd.append('-i')