[fix] "response doesn't start with the usual header"

fixes #25, fixes #69, fixes #75, fixes #81

also seen in issues: #56, #71

brother_ql/backends/generic.py

@@ -1,4 +1,6 @@
 
+from builtins import bytes
+
 import logging
 
 logger = logging.getLogger(__name__)

brother_ql/backends/pyusb.py

@@ -9,7 +9,7 @@ Install via `pip install pyusb`
 """
 
 from __future__ import unicode_literals
-from builtins import str
+from builtins import str, bytes
 
 import time