[3.8] gh-98433: Fix quadratic time idna decoding. (GH-99092) (GH-99222) (GH-99231)

There was an unnecessary quadratic loop in idna decoding. This restores
the behavior to linear.

(cherry picked from commit d315722564927c7202dd6e111dc79eaf14240b0d)
(cherry picked from commit a6f6c3a3d6f2b580f2d87885c9b8a9350ad7bf15)

Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>