Cap dnspython dependency at 1.16 on Python 2

The stable version of dnspython is 1.16.0, released on December 8, 2018.

Python 2.x support ends with the release of 1.16.0, unless there are critical bugs in 1.16.0. Future versions of dnspython will only support Python 3.

http://www.dnspython.org/

I think we might need to cap our dnspython req to <=1.16 on Python 2 to avoid installing incompatible versions.