[3.13] gh-134908: Protect `textiowrapper_iternext` with critical section (gh-134910) (gh-135040)

The `textiowrapper_iternext` function called `_textiowrapper_writeflush`, but did not
use a critical section, making it racy in free-threaded builds.
(cherry picked from commit 44fb7c361cb24dcf9989a7a1cfee4f6aad5c81aa)

Co-authored-by: Duane Griffin <duaneg@dghda.com>