TST: numpy.str does not exist in numpy 1.25.0.dev

[pllim]

This causes test errors in io.ascii.

https://anaconda.org/scipy-wheels-nightly/numpy

astropy/astropy/io/ascii/core.py

Line 1146 in 7460963

col.converters.insert(0, convert_numpy(numpy.str))

>>> import numpy
>>> numpy.__version__
'1.25.0.dev0+11.ga872fd73e'
>>> numpy.str
FutureWarning: In the future `np.str` will be defined as the corresponding NumPy scalar.
(This may have returned Python scalars in past versions.
  numpy.str
---------------------------------------------------------------------------
.../numpy/__init__.py:284, in __getattr__(attr)
    281     from .testing import Tester
    282     return Tester
--> 284 raise AttributeError("module {!r} has no attribute "
    285                      "{!r}".format(__name__, attr))

AttributeError: module 'numpy' has no attribute 'str'