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

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.