Group is missing tasks

[Mulugruntz]

I noticed that canvas.group is using self.tasks a bit everywhere, but it's never declared in any __init__ (I might have missed it though). And the stubs don't declare it either. That has the effect of triggering mypy when doing mygroup.tasks.

error: "group" has no attribute "tasks"  [attr-defined]

I'm not sure if this is supposed to be a stub-only fix (I could do it) or if it highlights a deeper potential bug, where we could have branches that would raise AttributeError (I'll need some help, and a ticket would be opened on celery repo).