Python3: AESEncryptedField attempts to combine strings and bytes
This is only an error in Python 3.

In [this line](https://github.com/coleifer/peewee/blame/master/playhouse/fields.py#L320), the AESEncryptedField attempts to concatenate the `key` (a `bytes` object) with `'  '` (a `str`). This raises an error since the operation is not allowed.