DOC/BLD: Pinning sphinx to 1.5, as 1.7 has been released and it's incompatible with vendored numpydoc
- [X] closes #19742
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks Ahh this should have gone in `environment-dev.yaml` and been auto-generated by `scripts/convert_deps.py`. Can you make another PR @datapythonista? ahh that's right. If the performance problems are solved with Sphinx 1.7, I would rather do the small fix in our vendored numpydoc version instead of keep pinning sphinx to 1.5. 
I am trying to test this in https://github.com/pandas-dev/pandas/pull/19687, but the problem it keeps installing sphinx 1.6 because it is not yet available on defaults. Will change it temporarily to install from conda-forge to test it. Updated the other PR, let's see what it does: https://travis-ci.org/pandas-dev/pandas/jobs/343064791 I reverted this in https://github.com/pandas-dev/pandas/pull/19687, and the last commit there tested sphinx 1.7 on travis, and it ran without failure and without time-out. So it seems we are good for sphinx 1.7.