"from_dtype()" now supports the variable-width string dtype
"numpy.dtypes.StringDType", generating arbitrary strings via "text()"
(issue #4039).
Additionally, passing a dtype class such as "np.dtypes.StringDType"
where an instance like "np.dtypes.StringDType()" was expected now
raises a clear error, rather than the previous confusing message (or
silent coercion to the object dtype in "arrays()").
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