gh-117722: Fix Stream.readuntil with non-bytes buffer objects (#117723)

gh-16429 introduced support for an iterable of separators in
Stream.readuntil. Since bytes-like types are themselves iterable, this
can introduce ambiguities in deciding whether the argument is an
iterator of separators or a singleton separator. In gh-16429, only 'bytes'
was considered a singleton, but this will break code that passes other
buffer object types.

Fix it by only supporting tuples rather than arbitrary iterables.

Closes gh-117722.