Handle MIME encoded-word in DKIM-Signature headers

As recently found in patatt [1], mail gateways and archivers may mangle
headers like DKIM-Signature if they are sent as an excessively long
line. An example of this occuring was found when the DKIM-Signature
header generated by Microsoft Office 365 collided with the
header re-encoding performed by lists.sr.ht when generating mbox
archive files. This encoding causes dkim.verify() to fail.

The Python email.header module provides the decode_header() and
make_header() functions which can be used to handle MIME encoded-word
syntax or other header manglings which may occur. Fixing up the header
content using these functions before calling dkim.verify() allows the
verification to succeed.

[1]: https://lore.kernel.org/tools/20210531140539.7630-1-paul@pbarker.dev/

Signed-off-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210607100252.8253-2-paul@pbarker.dev

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