Project should declare extra dependency on dnspython

[jaraco]

As I was working through the dependencies for Formencode, I discovered that the dependency for DNS has changed over time, but has been undeclared except in documentation.

It would be better if Formencode could declare its dependency in the package metadata using 'extras'. I suggest the extra name 'DNS' or 'dns', such that if a package requires FormEncode with the DNS validation functionality, it will require formencode[dns], and thereby rely on FormEncode to declare its dependency on the necessary libraries to support the DNS validation.

In this way, the client packages won't need to themselves monitor and maintain the dependency declaration for the package which is otherwise only used by FormEncode.

Any reason you wouldn't accept a PR to this end?

Also, I note that the tests currently rely on dnspython or dnspython3 depending on the version of Python used. Is that the recommended configuration or is dnspython 1.3, which supports Python 3.3+, suitable?