Support formatting python3 scripts

Use vpython or vpython3 to invoke yapf depending on the shebang line for
each file.

Bug: 1062053
Change-Id: I497fa48c08aa6052a2b06df7cc7f18e2154ab898
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2241792
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>

git_cl.py

@@ -4931,7 +4931,13 @@ def CMDformat(parser, args):
       if not yapf_style:
         yapf_style = 'pep8'
 
-      cmd = [yapf_tool, '--style', yapf_style, f]
+      with open(f, 'r') as py_f:
+        if 'python3' in py_f.readline():
+          vpython_script = 'vpython3'
+        else:
+          vpython_script = 'vpython'
+
+      cmd = [vpython_script, yapf_tool, '--style', yapf_style, f]
 
       has_formattable_lines = False
       if not opts.full: