mail/py-dkimpy: Update to 0.7.0

Additional port changes:

• DISTVERSION
• Added dependencies:
  • mail/py-authres
  • mail/py-milter (unfortunately py27-only)
  • security/py-pynacl
• Replaced dependencies:
  • dns/py-dns -> dns/py-dnspython (allows py36 now)
• Added USE_PYTHON=concurrent.
• Added patch fixing manpage path.

PR: 225742
Submitted by: Sebastian Schwarz <seschwar@gmail.com>
Approved by: Kevin Golding <ports@caomhin.org> (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D14293