Update docutils to 0.16

Python docutils https://pypi.org/project/docutils/0.16/ was released on 2020-01-12, while using Release 0.13.1 (!3042 (merged)), released on 2016-12-09.

Release note

http://docutils.sourceforge.io/RELEASE-NOTES.html

Relevant issues

Edited Mar 17, 2020 by Takuya Noguchi