Fix AESEncryptField for python3
Related to: https://github.com/coleifer/peewee/issues/1262 The tests for the playhouse extra_fields are not run automatically. Is there a particular reason? Will all those byte literals be compatible with Python 2.6? See comments on #1262 and #1264 
In python3 you can't create a bytes string with bytes(' ') because you need to specify an encoding.

```
2017-05-05 22:34:43,025 ERROR      File "/home/moutix/.virtualenvs/stepmania-server/lib/python3.5/site-packages/playhouse/fields.py", line 320, in get_cipher
    key = key + bytes(' ') * (32 - len(key))

2017-05-05 22:34:43,025 ERROR    Uncaught exception: string argument without an encoding
```