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

[miss-islington]

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) [email protected] Co-authored-by: Gregory P. Smith [email protected]

• Issue: gh-98433

[miss-islington]

Sorry, I can't merge this PR. Reason: You're not authorized to push to this branch. Visit https://docs.github.com/articles/about-protected-branches/ for more information..

[vstinner]

@ambv: Would you mind to merge this security fix? It's already in other branches, including 3.7 ;-)