3.13.0a6 breaks asyncio.Stream.readuntil with bytearray separator

[bmerry]

Bug report

Bug description:

As discussed in #81322, the change I made in #16429 breaks asyncio.Stream.readuntil when used with iterable buffer-object types other than bytes (such as bytearray) because they're incorrectly interpreted as an iterable of separators.

I've got a patch ready; I'm just filing this bug to be able to reference it.

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

Linux

Linked PRs

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