'Variable.update_time' fails where system clock is not UTC

[tseaver]

$ nox -s unit-3.9
...
_______________ TestConfig.test_get_variable_w_alternate_client ________________

self = <unit.test_config.TestConfig testMethod=test_get_variable_w_alternate_client>

    def test_get_variable_w_alternate_client(self):
        from google.cloud._helpers import _rfc3339_to_datetime
    
        VARIABLE_NAME = "my-variable/abcd"
        VARIABLE_PATH = "%s/variables/%s" % (self.CONFIG_PATH, VARIABLE_NAME)
        RESOURCE = {
            "name": VARIABLE_PATH,
            "value": "bXktdmFyaWFibGUtdmFsdWU=",  # base64 my-variable-value
            "updateTime": "2016-04-14T21:21:54.5000Z",
            "state": "VARIABLE_STATE_UNSPECIFIED",
        }
        conn1 = _Connection()
        CLIENT1 = _Client(project=self.PROJECT, connection=conn1)
        conn2 = _Connection(RESOURCE)
        CLIENT2 = _Client(project=self.PROJECT, connection=conn2)
        config = self._make_one(client=CLIENT1, name=self.CONFIG_NAME)
    
        variable = config.get_variable(VARIABLE_NAME, client=CLIENT2)
    
        self.assertEqual(variable.name, VARIABLE_NAME)
        self.assertEqual(variable.full_name, VARIABLE_PATH)
>       self.assertEqual(
            variable.update_time, _rfc3339_to_datetime(RESOURCE["updateTime"])
        )
E       AssertionError: datetime.datetime(2016, 4, 15, 1, 21, 54, 500000, tzinfo=datetime.timezone.utc) != datetime.datetime(2016, 4, 14, 21, 21, 54, 500000, tzinfo=datetime.timezone.utc)

tests/unit/test_config.py:203: AssertionError
...

Even though it knows it is unmarshalling a UTC timestamp, the code mistakenly juggles with offsetting the time, when only milliseconds / microseconds are passed.