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Here is a patch that makes the output consistently (bytes, string) pairs. This is definitely a potential backward compatibility issue, but in general code which compensates for the old behavior should work fine with the new behavior, since it was always possible to get a (bytes, None) tuple back as a result, so most code should be handling that case correctly already.
IMO this change is nevertheless worthwhile; especially since if the patch in issue 4661 is accepted decode_header can be enhanced so that it will provide a way to obtain the bytes version of a header containing (RFC invalid) non-ASCII bytes.
Note that this breaks one of the tests in nttplib, so backward compatibility really is an issue, unfortunately. I think nttplib's use case can be satisfied via the issue 4661 patch coupled with the decode_header bytes-recovery enhancement.