pmb.helpers.pmaports: Annotate return type for get_list() as list (MR 2431)

I'm not sure why this was annotated as a Sequence rather than list. The
function clearly returns a list, not a Sequence, and the commit that
originally did it[1] provides no justification. This causes problems
with mypy checks in other places, so let's just annotate this as
returning a list.

 [1]: 198f302a