Numpy arrays of `str` dtype are not converted to Arrays

A `np.array` with `dtype=str`, when passed to Julia remains a `PyObject`, instead of becoming some useful `Array{String}`. Here's a minimal example:

```julia
julia> using PyCall

julia> py"""
import numpy as np
"""

julia> py"""np.array(["a"], dtype=str)"""
PyObject array(['a'], dtype='

Start by reproducing the minimal Julia/Python example with a NumPy array using dtype=str, then inspect PyCall's NumPy array conversion path. Done means the value is converted to a Julia Array{String} rather than remaining a PyObject.

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