DOC: Add dateutil to intersphinx
Looks like we'ree using `dateutil` items in the `See Also` section, for example:
https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.tseries.offsets.DateOffset.html

But, for sphinx to generate the links, we need `dateutil` configured as an intersphinx item in `doc/source/conf.py`.