…rant detach (#159600)

[3.15] gh-157364: Fix use-after-free in io.TextIOWrapper during reentrant detach (#159591)

* gh-143008: Fix Null pointer dereferences in TextIOWrapper underlying stream access (#145957)

TextIOWrapper keeps its underlying stream in a member called
`self->buffer`. That stream can be detached by user code, such as custom
`.flush` implementations resulting in `self->buffer` being set to NULL.
The implementation often checked at the start of functions if
`self->buffer` is in a good state, but did not always recheck after
other Python code was called which could modify `self->buffer`.

The cases which need to be re-checked are hard to spot so rather than
rely on reviewer effort create better safety by making all self->buffer
access go through helper functions.

Thank you yihong0618 for the test, NEWS and initial implementation in
gh-143041.



(cherry picked from commit db4b194)

* gh-154523: Fix data-race in `TextIOWrapper.detach()` (#154565)

(cherry picked from commit 8440a6a)

* gh-157364: Fix use-after-free in io.TextIOWrapper during reentrant detach (#157370)


(cherry picked from commit e19dc47)

---------





(cherry picked from commit ea1312b)

Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>
Co-authored-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: Brij Kapadia <97006829+brijkapadia@users.noreply.github.com>
Co-authored-by: Shamil <ashm.tech@proton.me>