tests: make pipes / PTYs non-inheritable in test-stdio.py A following patch requires that to test closing the receiving end of the pipe / PTYs. Even for existing tests, it might be safer to make the lifetime of the pipes / PTYs as short as possible.

util: enhance `nullcontextmanager` to be able to return __enter__ result This makes its functionality and signature equivalent to Python 3.7’s contextlib.nullcontext().

windows: augment docstring of `winstdout` class