gh-137335: Fix unlikely name conflicts for named pipes in multiprocessing and asyncio on Windows (#137389)

Since os.stat() raises an OSError for existing named pipe "\\.\pipe\...",
os.path.exists() always returns False for it, and tempfile.mktemp() can
return a name that matches an existing named pipe.

So, tempfile.mktemp() cannot be used to generate unique names for named
pipes. Instead, CreateNamedPipe() should be called in a loop with
different names until it completes successfully.