[Python] pa.array() does not handle list of dicts with bytes keys correctly under python3

It creates sub-arrays with nulls filled, instead of the provided values.

$ python

Python 3.6.8 (default, Jan 3 2019, 03:42:36)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyarrow as pa
>>> pa._version_
'0.15.0'
>>> a = pa.array([{b"a": [1, 2, 3]}])
>>> a
<pyarrow.lib.StructArray object at 0x7fdcb4c28168>
– is_valid: all not null
– child 0 type: list<item: int64>
[
null
]
>>> a = pa.array([{"a": [1, 2, 3]}])
>>> a
<pyarrow.lib.StructArray object at 0x7fdcb4c28108>
– is_valid: all not null
– child 0 type: list<item: int64>
[
[
1,
2,
3
]
]

It works under python2.