gh-118138: Adds test for multithreaded writes to files.#119107

elistevens wants to merge 4 commits intopython:mainpython/cpython:mainfrom

elistevens:estevens/gh-118138/textiowrapper_writeflushelistevens/cpython:estevens/gh-118138/textiowrapper_writeflushCopy head branch name to clipboard