Return tuple from SequenceParser and OptionalParser

[drhagen]

Right now, SequenceParser and OptionalParser return Python lists. This was originally done because [] is syntactically easier to work with than (). However, these are not really intended to be mutable sequences. This plays badly with typing, which has no concept of "a list of length 2 containing a string followed by an int", but does have tuple[str, int].

This is a breaking change, though not massively so because, at least for me, the outputs are typically splatted in a dataclass, destructured, concatenated with each other, length-checked, sliced, or indexed, all of which will work unchanged with tuples.