bpo-45453: Fix test_embed.StdPrinterTests (GH-28916) (GH-28917)

test_embed.StdPrinterTests now always use the file descriptor 1 for
stdout, rather than using sys.__stdout__.fileno().
PyFile_NewStdPrinter() does crash if the argument is not 1 or 2.

Fix also a few pyflakes warnings: remove unused import and variables.
(cherry picked from commit 678433f25e0d08dad7edf72be8f0cf9420e4ed2c)

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