Plot answer tests are failing randomly on python3

[ngoldbaum]

For some reason a subset of the plot answer tests are failing every few test runs on python3. We should try to figure out why that's happening.

The solution might be to simply update matplotlib on the test machine as we're currently running matplotlib 1.5.1. That will require coordination with an accompanying bump in the plot answers.

[ngoldbaum]

Here's what it looks like in Jenkins when this happens:

[ngoldbaum]

We updated the doc builder to use python 3.6 and matplotlib 2.2 but it looks like this just hapenned again:

https://tests.yt-project.org/job/yt_py3_git/272/

That means this is likely due to a real bug in yt that is causing some small amount of random jitter in the position of contours drawn by matplotlib on yt plots.

[ngoldbaum]

remilestoned for 4.0 because we don't have a way to reliably trigger this behavior.