StringIO does not report observed newline types #8513

io.StringIO does not report newline types through its newlines attribute.

import io

stream = io.StringIO(newline=None)
stream.write("a\nb\r\nc\rd")

print(stream.newlines)

CPython returns:

("\r", "\n", "\r\n")

RustPython currently inherits _TextIOBase.newlines, which returns None.

The same behavior is covered by the existing expected-failure tests in Lib/test/test_memoryio.py.

AI assistance: Codex:gpt-5.6-sol (research and drafting; reviewed and edited by the contributor).