mail/py-afew: update to 1.3.0

[yuri]

Summary

See https://github.com/afewmail/afew/releases/tag/1.3.0 for
the release notes. Most importantly mail/py-dkimpy is a new
dependency while devel/py-subprocess32 is no longer needed.

Furthermore the format of the installed documentation was
changed from manual pages to HTML. While a manpage is usually
preferable the one generated by Sphinx doesn't fit the manpage
format at all and isn't very readable. HTML is a much better
fit for afew's documentation. Hence the MANPAGES option was
renamed to DOCS.

Test Plan

$ portlint -AC
looks fine.
# poudriere testport -j FreeBSD:11:amd64 mail/py-afew
(everything looks ok)