[3.12] gh-76511: Fix email.Message.as_string() for non-ASCII message with ASCII charset (GH-116125) (GH-116364)

(cherry picked from commit f97f25ef5dfcdfec0d9a359fd970abd139cf3428)

Co-authored-by: Serhiy Storchaka <[email protected]>

CPython-sync-commit-latest: 85c32ad9fd3780d8cc68a692c396cc840c093d69

Author: github-actions[bot]