[code-health] Run XML pretty_print.py with vpython3

Bug: 942720
Change-Id: I281a2e39efe653d37fb115bf99f7680b57073cb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3900835
Commit-Queue: Fabrice de Gans <fdegans@chromium.org>
Auto-Submit: Fabrice de Gans <fdegans@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

git_cl.py

@@ -5476,7 +5476,7 @@ def CMDformat(parser, args):
 
       tool_dir = os.path.join(top_dir, xml_dir)
       pretty_print_tool = os.path.join(tool_dir, 'pretty_print.py')
-      cmd = ['vpython', pretty_print_tool, '--non-interactive']
+      cmd = ['vpython3', pretty_print_tool, '--non-interactive']
 
       # If the XML file is histograms.xml or enums.xml, add the xml path to the
       # command as histograms/pretty_print.py now needs a relative path argument