Add check and fix for python3 unicode assertion

trello/customfield.py

@@ -2,9 +2,14 @@
 from __future__ import with_statement, print_function, absolute_import
 
 import time
+import sys
 from trello import TrelloBase
 from trello.compat import force_str
 
+is_python3 = sys.version_info.major == 3
+if is_python3:
+	unicode = str
+
 
 class CustomFieldDefinition(TrelloBase):
 	"""