yapf doesn't check python3 format correctly

It seems the format check is not done using the python3 syntax/features. That might be due to the fact that is the system yapf isntallation in /usr/local/bin/yapf what is being used by the CI to do it, as highlighted in

WARNING:root:could not format u'Hlt/Hlt2Conf/python/Hlt2Conf/lines/strange/two_body.py': Command '['/usr/local/bin/yapf']' returned non-zero exit status 1
Traceback (most recent call last):
  File "/usr/local/bin/yapf", line 10, in <module>
    sys.exit(run_main())
  File "/usr/local/lib/python2.7/dist-packages/yapf/__init__.py", line 317, in run_main
    sys.exit(main(sys.argv))
  File "/usr/local/lib/python2.7/dist-packages/yapf/__init__.py", line 187, in main
    verify=args.verify)
  File "/usr/local/lib/python2.7/dist-packages/yapf/yapflib/yapf_api.py", line 129, in FormatCode
    tree = pytree_utils.ParseCodeToTree(unformatted_source)
  File "/usr/local/lib/python2.7/dist-packages/yapf/yapflib/pytree_utils.py", line 127, in ParseCodeToTree
    raise e
  File "<unknown>", line 69
    *,
     ^
SyntaxError: invalid syntax

since lb-format is called with the python version installed in the system

python lb-format --format-patch apply-formatting.patch origin/${TARGET_BRANCH}

Job #16082098 failed for 5e3e7087