Always have tasks_list be a list, not potentially some fancier iterable.
Bypass mypy's somewhat legitimate complaint about REFERENCE and DRIVER in
task_class: they could potentially be instance attributes, but we rely on
them being class attributes. Python does normally guarantee their existence
as class attributes (unless a derived class explicitly deletes them), but
they could be overridden by an instance attribute; that's just something
we don't do, so the class attribute's value is legitimate. We can't
expect mypy to know that, so work around its complaint.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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