packaging: add docutils as dependency
The previous commit revealed that attempting to run
`python setup.py build_doc` from the packaging virtualenv
failed due to missing docutils package. We didn't notice
this before because py2exe Windows packaging appears to
use a Python from another virtualenv (which does include
docutils) to invoke setup.py. I discovered this as part
of implementing packaging outside of that virtualenv
environment.
Differential Revision: https://phab.mercurial-scm.org/D8470