Fixes issue #17488: Change the subprocess.Popen bufsize parameter default value

from unbuffered (0) to buffering (-1) to match the behavior existing code
expects and match the behavior of the subprocess module in Python 2 to avoid
introducing hard to track down bugs.

diff --cc Doc/library/subprocess.rst
Simple merge

diff --cc Lib/subprocess.py
Simple merge

diff --cc Lib/test/test_subprocess.py
Simple merge

diff --cc Misc/NEWS
Simple merge