bpo-45195: Fix test_readline.test_nonascii() (GH-28329) (GH-28333)

Fix test_readline.test_nonascii(): sometimes, the newline character
is not written at the end, so don't expect it in the output.
(cherry picked from commit 797c8eb9ef511f0c25f10a453b35c4d2fe383c30)

Co-authored-by: Victor Stinner <vstinner@python.org>