Make internal `Peekable` more robust or switch to `more-itertools`

[shsms]

The internal peekable implementation doesn't support having None as a valid value (for example Peekable[int | None] will not work). We could fix it, but there is also an implementation in more-itertools which seems popular and well maintained, so we could just switch to using that one.

Originally posted by @llucax in #1295 (comment)

[shsms]

Now that I think about it, I think I did try a third party Peekable, and it didn't work for whatever reason, because they were trying to do something clever to the iterable, rather than just wrap the thing and remember the latest item.

Anyway, can try again and document if there are issues next time.