ECSV reader fails on datatype=object column created with astropy < 4.3 #11803

# %ECSV 0.9
# ---
# datatype:
# - {name: source, datatype: object}
# - {name: obstime, datatype: string}
# - {name: ra, datatype: float64}
# - {name: dec, datatype: float64}
# - {name: pa_v3, datatype: float64}
# schema: astropy-2.0
source obstime ra dec pa_v3
None 2016-01-18T15:25:23.776 133.3599835511599 -14.886980131752205 -161.3183607231063

In [1]: import astropy

In [2]: print(astropy.__version__)
4.3.dev1662+gdf850cb7c

In [4]: from astropy.table import Table

In [5]: table = Table.read("jwst/lib/tests/data/v1_calc_truth.ecsv")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-2e4749b2facf> in <module>
----> 1 table = Table.read("jwst/lib/tests/data/v1_calc_truth.ecsv")

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/table/connect.py in __call__(self, *args, **kwargs)
     59         descriptions = kwargs.pop('descriptions', None)
     60 
---> 61         out = registry.read(cls, *args, **kwargs)
     62 
     63         # For some readers (e.g., ascii.ecsv), the returned `out` class is not

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/io/registry.py in read(cls, format, cache, *args, **kwargs)
    525 
    526         reader = get_reader(format, cls)
--> 527         data = reader(*args, **kwargs)
    528 
    529         if not isinstance(data, cls):

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/io/ascii/connect.py in io_read(format, filename, **kwargs)
     16         format = re.sub(r'^ascii\.', '', format)
     17         kwargs['format'] = format
---> 18     return read(filename, **kwargs)
     19 
     20 

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/io/ascii/ui.py in read(table, guess, **kwargs)
    367         else:
    368             reader = get_reader(**new_kwargs)
--> 369             dat = reader.read(table)
    370             _read_trace.append({'kwargs': copy.deepcopy(new_kwargs),
    371                                 'Reader': reader.__class__,

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/io/ascii/core.py in read(self, table)
   1336 
   1337         # Get the table column definitions
-> 1338         self.header.get_cols(self.lines)
   1339 
   1340         # Make sure columns are valid

~/miniconda3/envs/dev/lib/python3.9/site-packages/astropy/io/ascii/ecsv.py in get_cols(self, lines)
    184             col.dtype = header_cols[col.name]['datatype']
    185             if col.dtype not in ECSV_DATATYPES:
--> 186                 raise ValueError(f'datatype {col.dtype!r} of column {col.name!r} '
    187                                  f'is not in allowed values {ECSV_DATATYPES}')
    188 

ValueError: datatype 'object' of column 'source' is not in allowed values ('bool', 'int8', 'int16', 'int32', 'int64', 'uint8', 'uint16', 'uint32', 'uint64', 'float16', 'float32', 'float64', 'float128', 'string')

>>> import platform; print(platform.platform())
macOS-10.15.7-x86_64-i386-64bit
>>> import sys; print("Python", sys.version)
Python 3.9.5 (default, May 18 2021, 12:31:01) 
[Clang 10.0.0 ]
>>> import numpy; print("Numpy", numpy.__version__)
Numpy 1.20.3
>>> import astropy; print("astropy", astropy.__version__)
astropy 4.3.dev1662+gdf850cb7c
>>> import scipy; print("Scipy", scipy.__version__)
Scipy 1.6.3
>>> import matplotlib; print("Matplotlib", matplotlib.__version__)
Matplotlib 3.4.2