Detchar unit test failure in frame reading: astropy.io.registry.IORegistryError: Format could not be identified

The following error occurs in the unit tests when we try to read a state vector from a frame file. The error started with gwpy 0.14.1. Rolling back to gwpy 0.14.0 fixes the issue.

CC @duncanmmacleod, @geoffrey.mo.

=================================== FAILURES ===================================
_________________________ test_handle_create_grb_event _________________________

mock_create_event = <MagicMock name='create_event' id='5115675088'>
mock_get_event = <MagicMock name='get_event' id='5119847392'>
mock_write_log = <MagicMock name='writeLog' id='5119773944'>
mock_json = <MagicMock name='dqr_json' id='5119823264'>

    @patch('gwcelery.tasks.detchar.dqr_json', return_value='dqrjson')
    @patch('gwcelery.tasks.gracedb.client.writeLog')
    @patch('gwcelery.tasks.gracedb.get_event', return_value={
        'graceid': 'E1', 'gpstime': 1, 'instruments': '', 'pipeline': 'Fermi',
        'extra_attributes': {'GRB': {'trigger_duration': 1}}})
    @patch('gwcelery.tasks.gracedb.create_event')
    def test_handle_create_grb_event(mock_create_event, mock_get_event,
                                     mock_write_log, mock_json):
        text = resource_string(__name__, 'data/fermi_grb_gcn.xml')
>       external_triggers.handle_grb_gcn(payload=text)

gwcelery/tests/test_tasks_external_triggers.py:20: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/celery/app/task.py:375: in __call__
    return self.run(*args, **kwargs)
gwcelery/tasks/external_triggers.py:97: in handle_grb_gcn
    detchar.check_vectors(event, event['graceid'], start, end)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/celery/app/task.py:375: in __call__
    return self.run(*args, **kwargs)
gwcelery/tasks/detchar.py:342: in check_vectors
    start, end, globals()[bits]))
gwcelery/tasks/detchar.py:266: in check_vector
    bits=bits)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/gwpy/timeseries/statevector.py:688: in read
    return super(StateVector, cls).read(source, *args, **kwargs)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/gwpy/timeseries/core.py:310: in read
    return timeseries_reader(cls, source, *args, **kwargs)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/gwpy/timeseries/io/core.py:45: in read
    return io_read_multi(joiner, cls, source, *args, **kwargs)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/gwpy/io/mp.py:73: in read_multi
    kwargs['format'] = get_read_format(cls, path, (source,) + args, kwargs)
../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/gwpy/io/registry.py:95: in get_read_format
    return get_format('read', cls, filepath, fileobj, args, kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

mode = 'read', cls = <class 'gwpy.timeseries.statevector.StateVector'>
path = None, fileobj = None, args = ([], 'H1:DMT-DQ_VECTOR')
kwargs = {'bits': <Bits(1: 'NO_OMC_DCPD_ADC_OVERFLOW'
      2: 'NO_DMT-ETMY_ESD_DAC_OVERFLOW',
      channel=<Channel("DMT-DQ_VECTOR", None) at 0x12f8679b0>,
      epoch=None)>, 'end': 4, 'start': -1}

    def _get_valid_format(mode, cls, path, fileobj, args, kwargs):
        """
        Returns the first valid format that can be used to read/write the data in
        question.  Mode can be either 'read' or 'write'.
        """
    
        valid_formats = identify_format(mode, cls, path, fileobj, args, kwargs)
    
        if len(valid_formats) == 0:
            format_table_str = _get_format_table_str(cls, mode.capitalize())
            raise IORegistryError("Format could not be identified.\n"
                                  "The available formats are:\n"
>                                 "{0}".format(format_table_str))
E           astropy.io.registry.IORegistryError: Format could not be identified.
E           The available formats are:
E              Format    Read Write Auto-identify
E           ------------ ---- ----- -------------
E                    csv  Yes   Yes           Yes
E                    gwf  Yes   Yes           Yes
E           gwf.framecpp  Yes   Yes            No
E           gwf.lalframe  Yes   Yes            No
E                   hdf5  Yes   Yes           Yes
E              hdf5.losc  Yes    No            No
E                    txt  Yes   Yes           Yes

../../.local/share/virtualenvs/gwcelery-qtf_ZR8h/lib/python3.6/site-packages/astropy/io/registry.py:595: IORegistryError