Don't use `map` here - it doesn't make a list on Python 3.

celery/canvas.py

@@ -607,7 +607,7 @@ def _maybe_group(tasks):
     elif isinstance(tasks, Signature):
         tasks = [tasks]
     else:
-        tasks = map(signature, regen(tasks))
+        tasks = [signature(t) for t in regen(tasks)]
     return tasks