BUG: Unpacking PY2 msgpack in PY3
In #10686, I should have made all the strings in `encode` Unicode strings.  Now `'abc'` packed in P2 becomes (or rather remains as) `b'abc'` when unpacked in P3.  This I think is the desired behavior (bytes remain as bytes and text remains as text), but it causes errors in `decode`, because, for example, `'typ'` (`==u'type'` in P2) is expected while `b'typ'` (`=='typ'` in P2) is the key.

Reading in the other direction is fine because P2 is more tolerant of these things.

To reproduce this, 

```
(P2) python generate_legacy_storage_files.py your_dir msgpack
(P3) pandas.read_msgpack(the_file_just_created)
```
