gh-88435: smtplib make AUTH work with non-ASCII user/pw

[jrlevine]

EAI mail systems can have UTF-8 usernames and passwords. This makes AUTH use the same encoding as the rest of smtplib

https://bugs.python.org/issue44269

• Issue: smtplib AUTH command doesn't handle EAI arguments #88435

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[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

Please add tests and a NEWS entry.

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[zware]

This looks like a reasonable change to me, but unfortunately the PR was opened against the wrong branch (3.9 vs main). It will need to be recreated, but the change still applies cleanly to main.