[3.12] gh-127655: Ensure `_SelectorSocketTransport.writelines` pauses the protocol if needed (GH-127656) (#127664)

gh-127655: Ensure `_SelectorSocketTransport.writelines` pauses the protocol if needed (GH-127656)

Ensure `_SelectorSocketTransport.writelines` pauses the protocol if it reaches the high water mark as needed.
(cherry picked from commit e991ac8f2037d78140e417cc9a9486223eb3e786)

Co-authored-by: J. Nick Koston <nick@koston.org>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>