TST: dtype for BigQuery TIMESTAMP unexpectedly using datetime64[ns, UTC] dtype

$ pytest 'tests/system/test_gbq.py::TestReadGBQIntegration::test_return_correct_types[env-current_timestamp()-datetime64[ns]]'
=============================== test session starts ===============================
platform darwin -- Python 3.6.4, pytest-4.2.0, py-1.8.0, pluggy-0.8.1
rootdir: /Users/swast/src/pandas/pandas-gbq, inifile:
collected 1 item                                                                  

tests/system/test_gbq.py F                                                  [100%]

==================================== FAILURES =====================================
_ TestReadGBQIntegration.test_return_correct_types[env-current_timestamp()-datetime64[ns]] _

self = <tests.system.test_gbq.TestReadGBQIntegration object at 0x10c3277b8>
project_id = 'swast-scratch', expression = 'current_timestamp()'
type_ = 'datetime64[ns]'

    @pytest.mark.parametrize(
        "expression, type_",
        [
            ("current_date()", "<M8[ns]"),
            ("current_timestamp()", "datetime64[ns]"),
            ("current_datetime()", "<M8[ns]"),
            ("TRUE", bool),
            ("FALSE", bool),
        ],
    )
    def test_return_correct_types(self, project_id, expression, type_):
        """
        All type checks can be added to this function using additional
        parameters, rather than creating additional functions.
        We can consolidate the existing functions here in time
    
        TODO: time doesn't currently parse
        ("time(12,30,00)", "<M8[ns]"),
        """
        query = "SELECT {} AS _".format(expression)
        df = gbq.read_gbq(
            query,
            project_id=project_id,
            credentials=self.credentials,
            dialect="standard",
        )
>       assert df["_"].dtype == type_
E       AssertionError: assert datetime64[ns, UTC] == 'datetime64[ns]'
E        +  where datetime64[ns, UTC] = 0   2019-03-22 22:35:32.398261+00:00\nName: _, dtype: datetime64[ns, UTC].dtype

tests/system/test_gbq.py:392: AssertionError
============================ 1 failed in 2.68 seconds =============================

It’s odd that we explicitly specify the datetime64[ns] dtype, but it comes back as datetime64[ns, UTC] on the latest pandas version. I know to_dataframe from google-cloud-bigquery returns datetime objects with the UTC timezone, but I’d expect an explicit dtype of datetime64[ns] to take precedence.

[tswast]

Filed upstream at https://github.com/pandas-dev/pandas/issues/25843

[max-sixty]

Should we put this upstream? That second example is a good one…