DOC: test new sphinx 2 release
The docs are currently being built with sphinx 1.8.5 (see eg https://travis-ci.org/pandas-dev/pandas/jobs/518832177 for a recent build on master).

Sphinx has released 2.0.0 (http://www.sphinx-doc.org/en/master/changes.html#release-2-0-0-released-mar-29-2019), and it would be good to test our docs with this new release, and see if we need to make changes / report regressions to sphinx.

For somebody wanting to tackle this:
- test it locally to see if there are big problems with building the docs
- make a PR that ensures sphinx 2 is installed in the doc environment, so we can check the build log on travis (I am actually not fully sure why it is not yet picking up sphinx 2 on travis, since we don't pin the version in the [travis-36-doc.yaml file](https://github.com/pandas-dev/pandas/blob/a07ed594ec6a5befc967fb1b18244bbeb3bc2bf1/ci/deps/travis-36-doc.yaml#L36)