* Correct minimum version
see #2092
While this is needed because of an import just for typing, I see no reason not to bump the minimum version since this is a minor version difference since this is several minor version behind the latest 3.6, and there have been both security and performance improvements since.
That said, we need to be testing on our lowest supported version to ensure we don't have this happen again, right now our tests run on whatever Travis grabs for 3.6, which I assume is 3.6.6, but could be wrong.
* Update other mentions of min version to 3.6.2
* [V3 Docs] drop ffmpeg from CentOS install guide
* [V3 Install Docs] move all to Python 3.6
* Update the toctree
* Needed a blank line
* drop a .6 that wasn't needed
