GeezerVison

PR python#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 python#16429, only 'bytes'
was considered a singleton, but this will break code that passes other
buffer object types.

The Python library docs don't indicate what separator types were
permitted in Python <=3.12, but comments in typeshed indicate that it
would work with types that implement the buffer protocol and provide a
len(). To keep those cases working the way they did before, I've changed
the detection logic to consider any instance of collections.abc.Buffer
as a singleton separator.

There may still be corner cases where this doesn't do what the user
wants e.g. a numpy array of byte strings will implement the buffer
protocol and hence be treated as a singleton; but at least those corner
cases should behave the same in 3.13 as they did in 3.12.

Relates to python#81322.