chore(python_mono_repo): add support for grafeas

[parthea]

This PR fixes the issue where the owlbot post processor fails for grafeas because it runs a nox session which expects a directory google in the package root. See build log here. See the failure here.

See stack trace below

nox > black docs google tests noxfile.py setup.py
Usage: black [OPTIONS] SRC ...
Try 'black -h' for help.

Error: Invalid value for 'SRC ...': Path 'google' does not exist.
nox > Command black docs google tests noxfile.py setup.py failed with exit code 2
nox > Session format failed.
2023-10-29 14:42:48,408 synthtool [ERROR] > Failed executing nox -s format:

None
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/synthtool/synthtool/languages/python_mono_repo.py", line 263, in <module>
    owlbot_main(package_dir)
  File "/synthtool/synthtool/languages/python_mono_repo.py", line 251, in owlbot_main
    synthtool.shell.run(
  File "/synthtool/synthtool/shell.py", line 39, in run
    raise exc
  File "/synthtool/synthtool/shell.py", line 27, in run
    return subprocess.run(
  File "/usr/local/lib/python3.10/subprocess.py", line 524, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['nox', '-s', 'format']' returned non-zero exit status 1.