Fix spacing between parameter and parameter type in the API docs

[lesteve]

This can be quite confusing: e.g. it looks like the parameters are queuestr and projectstr in this screenshot:

• this is related to a change in sphinx 2. This was reported here in numpydoc: Sphinx 2.0 compatibility issue? numpy/numpydoc#215
• there has been a sphinx fix in Fix #5959: classifier delimiter is not shown sphinx-doc/sphinx#5976
• that sphinx_rtd_theme does not use basic.css. Since dask_sphinx_theme does extends sphinx_rtd_theme we are not benefitting from the sphinx fix. Hopefully this will get merged eventually Inherent sphinx base css readthedocs/sphinx_rtd_theme#747

The simplest fix I found was to add basic.css through setup(app) in conf.py

cc @TomAugspurger since i saw he had similar problems in dask-ml which he fixed by pinning sphinx < 2 (except that we can not do that on RTD, which is another story by itself ...)

This is the same screenshot as above from a locally generated doc:

[lesteve]

I am going to merge this one, let me know if you spot problems in the doc that may be related to this PR.

[guillaumeeb]

Thanks @lesteve! Noticed this too, this caused some missunderstanding in some issues. It's really nice to have it fixed!