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

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