Clean build environment produces warnings

[miketheman]

On current master (17a0ca1ee7d09744cdcd5e5c8a56822278651fb8)

make html produces this warning repeated frequently:

deprecation warning: io.FileInput() argument `handle_io_errors` is ignored since "Docutils 0.10 (2012-12-16)" and will soon be removed.reading sources... [

Environment is also pretty outdated, and could use a fresh coat of paint.

$ python --version
Python 2.7.15
$ pip list
Package          Version
---------------- -------
docutils         0.14
Jinja2           2.10
MarkupSafe       1.1.0
pip              18.1
Pygments         2.3.1
setuptools       40.6.3
Sphinx           1.2
sphinx-rtd-theme 0.1.5
wheel            0.32.3
$ pip list --outdated
Package          Version Latest Type
---------------- ------- ------ -----
Sphinx           1.2     1.8.3  wheel
sphinx-rtd-theme 0.1.5   0.4.2  wheel

So it appears that we likely want to update the baseline Sphinx installation, as well as move the entire project to Python 3, as Python 2 will end of life at the end of 2020 - see https://www.python.org/dev/peps/pep-0373/#update