*args argument should be a Sequence, not a list

[JukkaL]

The type of a *args argument is a list (a leftover from Alore), even though it is actually a tuple (i.e. a Sequence until we add support for TupleSequence). For example, this program is accepted even though it fails at runtime:

def f(*x: int) -> None:
    x.append(1)  # Ouch
f(1)