pynacl should be a fix dependency

[lukn]

  File "/whatever/is/my/funny/project.py", line xxx, in functionname
    valid = d.verify()
  File "/usr/lib/python3.6/site-packages/dkim/__init__.py", line 879, in verify
    return self.verify_sig(sig, include_headers, sigheaders[idx], dnsfunc)
  File "/usr/lib/python3.6/site-packages/dkim/__init__.py", line 674, in verify_sig
    pk, self.keysize, ktag = load_pk_from_dns(name, dnsfunc)
  File "/usr/lib/python3.6/site-packages/dkim/__init__.py", line 440, in load_pk_from_dns
    pk = nacl.signing.VerifyKey(pub[b'p'], encoder=nacl.encoding.Base64Encoder)
NameError: name 'nacl' is not defined

with more and more dkim users publishing elliptic curve keys it would be reasonable to add pynacl as a required dependency. Additionally, pynacl requires cffi as backend.

[Scott Kitterman (kitterman)]

I'm not sure I agree, particularly since adding something that needs to be compiled when installed via pip adds a lot of complexity. We should definitely have a better error message here though.

[Scott Kitterman (kitterman)]

Changes to make the error clearer committed. While I would encourage distributions to make pynacl a dependency (I did in the Debian package), I don't think it's appropriate for installs from pypi.

[Scott Kitterman (kitterman)]

2019-12-15 Version 1.0.1
    - Follow CNAMES when looking up key records when using DNS (pydns)
      (LP: #1856421)
    - Provide specialized error message when signing or verifying ed25519
      signatures and pynacl is not installed (LP: #1854475)