Move environment instructions to Python 3

[dtrudg]

Which Document page:

README.md / Makefile etc.

Expected results:

We recommend using Python 3 as the environment for creating docs.

Actual results:

We are recommending a Python 2.7 environment, which has less than 3 months of support left. (We provide a direct python.org link, and aren't recommending a distro version that will have distro support for longer than that).

[dtrudg]

Python 3 is now the present, and the future...