feat: Add support for Python 3.12

[gcf-owl-bot]

Source-Link: googleapis/synthtool@af16e6d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:bacc3af03bff793a03add584537b36b5644342931ad989e3ba1171d3bd5399f5

[parthea]

Wait for googleapis/python-pubsub#1025 because google-cloud-pubsub is used during presubmit testing https://source.cloud.google.com/results/invocations/f824025e-0009-4095-ae9d-e9dd2197c426/log

[parthea]

The failure is unrelated to the changes in this PR. It was fixed upstream in googleapis/python-pubsub@3db4ccc

tests/unit/pubsub_v1/publisher/test_publisher_client.py:639: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = , name = 'called_once'

    def __getattr__(self, name):
        if name in {'_mock_methods', '_mock_unsafe'}:
            raise AttributeError(name)
        elif self._mock_methods is not None:
            if name not in self._mock_methods or name in _all_magics:
                raise AttributeError("Mock object has no attribute %r" % name)
        elif _is_magic(name):
            raise AttributeError(name)
        if not self._mock_unsafe and (not self._mock_methods or name not in self._mock_methods):
            if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')) or name in _ATTRIB_DENY_LIST:
>               raise AttributeError(
                    f"{name!r} is not a valid assertion. Use a spec "
                    f"for the mock if {name!r} is meant to be an attribute.")
E               AttributeError: 'called_once' is not a valid assertion. Use a spec for the mock if 'called_once' is meant to be an attribute.

/usr/local/lib/python3.12/unittest/mock.py:663: AttributeError