DOC/BLD: Pinning sphinx to 1.5, as 1.7 has been released and it's incompatible with vendored numpydoc

[datapythonista]

• closes Building docs fails with the latest version of sphinx #19742
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jreback]

thanks

[TomAugspurger]

Ahh this should have gone in environment-dev.yaml and been auto-generated by scripts/convert_deps.py. Can you make another PR @datapythonista?

[jreback]

ahh that's right.

[jorisvandenbossche]

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 #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.

[jorisvandenbossche]

Updated the other PR, let's see what it does: https://travis-ci.org/pandas-dev/pandas/jobs/343064791

[jorisvandenbossche]

I reverted this in #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.