Issue with building using Python 3.8

[brynrhodes]

When using Python 3.8 to build with mkdocs, I ran into the following issue:

tornadoweb/tornado#2608

I addressed it by following the issue comment here:

tornadoweb/tornado#2608 (comment)

Not ideal, but it did get me working. Seems to be a relatively recent issue and the fix hasn't worked it's way through the packaging for mkdocs yet.