Fix generating docs for Readthedocs

[gdubicki]

• We need to use Python 3 as we need to install pypuppetdb to get its code
  parsed for autodoc
• Warnings in doc build mean that autodocs failed - this is critical,
  as docs will be incomplete, so break such build
• Fix a warning from doc build
• Update info how to build the docs locally

[coveralls]

Coverage remained the same at 83.549% when pulling d791668 on fix_readthedocs into d106ba4 on master.