chore(python): use python 3.10 for docs build

[gcf-owl-bot]

Source-Link: googleapis/synthtool@9ae0785
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:52210e0e0559f5ea8c52be148b33504022e1faef4e95fbe4b32d68022af2fa7e

[parthea]

Traceback (most recent call last):
  File "/tmpfs/src/github/python-ndb/.nox/docs/bin/sphinx-build", line 5, in 
    from sphinx.cmd.build import main
  File "/tmpfs/src/github/python-ndb/.nox/docs/lib/python3.10/site-packages/sphinx/cmd/build.py", line 25, in 
    from sphinx.application import Sphinx
  File "/tmpfs/src/github/python-ndb/.nox/docs/lib/python3.10/site-packages/sphinx/application.py", line 31, in 
    from sphinx.config import Config
  File "/tmpfs/src/github/python-ndb/.nox/docs/lib/python3.10/site-packages/sphinx/config.py", line 21, in 
    from sphinx.util import logging
  File "/tmpfs/src/github/python-ndb/.nox/docs/lib/python3.10/site-packages/sphinx/util/__init__.py", line 41, in 
    from sphinx.util.typing import PathMatcher
  File "/tmpfs/src/github/python-ndb/.nox/docs/lib/python3.10/site-packages/sphinx/util/typing.py", line 37, in 
    from types import Union as types_Union
ImportError: cannot import name 'Union' from 'types' (/usr/local/lib/python3.10/types.py)
nox > Command sphinx-build -W -T -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/ failed with exit code 1
nox > Session docs failed.