Docs build hangs at pylab_examples/system_monitor example

[jenshnielsen]

See https://travis-ci.org/matplotlib/matplotlib/jobs/71414788

I suspect this happens after the merge of #4506 That example still calls plt.ion()

[tacaswell]

But what is odd is that the tests passed on that commit: https://travis-ci.org/matplotlib/matplotlib/builds/71330399

[jenshnielsen]

Could it be a combination of this and the MEP12 done on the same example in #4648 ?

[tacaswell]

Closing as this is fixed.