gh-130148: Add redirect_stdout(..., per_thread: bool = False)

[Liam-DeVoe]

In #130148, there was discussion of what would be required to make redirect_stdout thread-aware. #130148 (comment) proposed adding a per_thread: bool = False argument. This PR implements that argument.

To support interleaving per_thread=False and per_thread=True, this PR uses a slightly more complicated design involving walking up the stack of context managers to find the first one applicable to its thread.

As an example motivating usage, making doctest thread-safe becomes a ~one line change with this PR: https://github.com/python/cpython/compare/main...Liam-DeVoe:cpython:redirect-stdout-doctest?expand=1.

I think there's been previous discussion of whether this should live in CPython or be proven out in another project first. I don't know exactly where people have fallen on this. I opened a previous version of this code against Quansight-Labs/pytest-run-parallel#167, but after discussion with @ngoldbaum it was unclear to us that pytest-run-parallel is the right place for this to live, rather than the standard library.

cc: @ngoldbaum @colesbury

• Issue: contextlib.redirect_stdout/stderr does not work with pytest-run-parallel #130148

[picnixz]

Please open a new issue for that. I believe that we also suggested this to be outside the stdlib so I do not think we want this for now.