Building fails

[zcorpan]

#145 is failing in CircleCI with this error:

make dirhtml
Running Sphinx v2.3.1

Warning, treated as error:
node class 'meta' is already registered, its visitors will be overridden
make: *** [Makefile:20: dirhtml] Error 2

Exited with code exit status 2

I could reproduce this same error locally on master also. Not sure what's up.

cc @mariestaver

[mariestaver]

@zcorpan It looks like this could be due to a known issue with Sphinx -- is this project using docutils-0.18? If so, we may need to downgrade to 0.17 to fix it (from sphinx-doc/sphinx#9841).