[Python] Conversion from Numpy object array to varsize binary unimplemented

>>> arr = np.array([b'xx'], dtype=np.object)
>>> pa.array(arr, type=pa.binary(2))
<pyarrow.lib.FixedSizeBinaryArray object at 0x7fe1ecaefa98>
[
  b'xx'
]
>>> pa.array(arr, type=pa.binary())
Traceback (most recent call last):
  File "<ipython-input-12-e40948b94b33>", line 1, in <module>
    pa.array(arr, type=pa.binary())
  File "array.pxi", line 177, in pyarrow.lib.array
  File "error.pxi", line 77, in pyarrow.lib.check_status
  File "error.pxi", line 85, in pyarrow.lib.check_status
ArrowNotImplementedError: /home/antoine/arrow/cpp/src/arrow/python/numpy_to_arrow.cc:1585 code: converter.Convert()
/home/antoine/arrow/cpp/src/arrow/python/numpy_to_arrow.cc:1098 code: compute::Cast(&context, *arr, type_, options, &casted)
/home/antoine/arrow/cpp/src/arrow/compute/kernels/cast.cc:1022 code: Cast(ctx, Datum(array.data()), out_type, options, &datum_out)
/home/antoine/arrow/cpp/src/arrow/compute/kernels/cast.cc:1009 code: GetCastFunction(*value.type(), out_type, options, &func)
No cast implemented from binary to binary