Add Sphinx and Python version support policy

[choldgraf]

This adds a rough policy for the versions of Python and Sphinx that we support (since these are our two major dependencies).

I proposes:

• Supporting Python versions with security support that are < 3.5 years old.
• Supporting the latest released Sphinx version that is > 6 months old. (unofficial support for older releases, but no promises/testing)

closes #567 closes #446 closes #510

[choldgraf]

cc @jorisvandenbossche and @damianavila who also commented in the issue this would close

[damianavila]

cc @jorisvandenbossche and @damianavila who also commented in the issue this would close

I am happy with the current policy as is, thanks for the cc @choldgraf!