DOC: Add dateutil to intersphinx

[datapythonista]

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.

[benjaminr]

Happy to have a look at this one.