Missing cryptography dependency when following CONTRIBUTING.md

[rdrey]

requirements.txt lists cryptography as a dependency, but setup.py doesn't. CONTRIBUTING.md doesn't instruct contributors to install requirements.txt.

This causes two unit test failures and one error (collecting tests/providers/test_googleclouddns.py) in Python 2.7.

We could move it to optional dependencies since only one provider uses it. Or we could add it to setup.py since it's already installed in py3 for many users.

[AnalogJ]

thats a great catch, something I completely overlooked.

I think it might make even more sense to change requirements.txt to just contain a dot (.)

That would make setup.py the single source of truth for all dependency versions.

Thoughts?

[rdrey]

Sounds good! (As long as lexicon doesn't need to pin dependency versions it might as well all be in setup.py.)