gh-122989: Replace “self.policy.linesep” with “linesep”

[Damien-Chen]

resolve #122989

• Issue: What’s the difference between “linesep” and “self.policy.linesep” in Lib.email.generator.Generator._write_headers? #122989

[mcepl]

LGTM

[picnixz]

@terryjreedy Do we backport it? (it was marked as a bugfix and not a feature)