gh-90738: fix logic in email.utils.unquote, previously it removed slashes in ov…

[bAndie91]

…erlapping sequences, eg \\\\" -> " instead of -> \\"

[the-knights-who-say-ni]

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[JelleZijlstra]

Could you add a test and create a bugs.python.org issue linked to this PR?

[bAndie91]

https://bugs.python.org/issue46580

[AlexWaygood]

https://bugs.python.org/issue46580

If you change the title of your PR to "bpo-46580: fix logic in email.utils.unquote", the bots will automatically link this PR to your BPO issue, and the relevant CI check will also go green :)

[JelleZijlstra]

Still needs a test.