Python3 support

[swalkinshaw]

This makes Trellis compatible with both Python 2 and 3. To use Python 3 though, Ansible >= 2.5 is required. However, since we already tell people to avoid 2.5.0, let's just say use 2.5.3 at minimum.

[fullyint]

I haven't reviewed yet. There may be some ideas in the old python-3-compat branch, e.g., in 9e83883

Here are some hastily pasted related notes I had from months ago...

• Users can override the dynamic determination of interpreter by simply defining somewhere ansible_python_interpreter var (e.g., per host, or more globally in group_vars). See Incorrect python interpreter on local connection  ansible/ansible#21311
• httplib (letsencrypt test acme challenges) taken from https://github.com/ansible/ansible/blob/e4efc191c5be078ead687553538f92d035946f1f/lib/ansible/module_utils/urls.py#L45-L49

I'll try to make time to review my old PY3 to-do notes and add them here.

[swalkinshaw]

Oh thanks, either I never knew about or forgot 😳

Looks like those changes are basically split 50/50 between Python on local and remote. This PR is definitely only for Python 3 on local machine.

Most of the changes are similar in concept, but my branch didn't seem to need a lot of them? Keep in mind I didn't test deploys yet.

[swalkinshaw]

Going to merge this as is. We'll finish Ubuntu 18.04 separately this is useful on its own.

@fullyint feel free to post any other notes you may have.