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='