[3.12] gh-144125: email: verify headers are sound in BytesGenerator

gh-144125: email: verify headers are sound in BytesGenerator
(cherry picked from commit 052e55e7d44718fe46cbba0ca995cb8fcc359413)

Co-authored-by: Seth Michael Larson <seth@python.org>
Co-authored-by: Denis Ledoux <dle@odoo.com>
Co-authored-by: Denis Ledoux <5822488+beledouxdenis@users.noreply.github.com>
Co-authored-by: Petr Viktorin <302922+encukou@users.noreply.github.com>
Co-authored-by: Bas Bloemsaat <1586868+basbloemsaat@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>