Don't use the str.lower() method, in Python 3 it's not a string.

topydo/lib/ListContextCommand.py

@@ -28,7 +28,7 @@ class ListContextCommand(Command):
         if not super(ListContextCommand, self).execute():
             return False
 
-        for context in sorted(self.todolist.contexts(), key=str.lower):
+        for context in sorted(self.todolist.contexts(), key=lambda s: s.lower()):
             self.out(context)
 
     def usage(self):

topydo/lib/ListProjectCommand.py

@@ -28,7 +28,7 @@ class ListProjectCommand(Command):
         if not super(ListProjectCommand, self).execute():
             return False
 
-        for project in sorted(self.todolist.projects(), key=str.lower):
+        for project in sorted(self.todolist.projects(), key=lambda s: s.lower()):
             self.out(project)
 
     def usage(self):