DOC: Sphinx 1.7 compatability

[TomAugspurger]

I thought I posted this when the beta was out, but apparently not. We have a bit of work to do.

• Remove vendored IPython sphinxext (should be OK with BUG sphinxext: ensure sys.stdout is used for logging (#10904) ipython/ipython#10907 released)
• Remove vendored numpydoc (work around Format attributes like parameters numpy/numpydoc#106 (comment))
• Check performance (better than 1.6, but still slower? Considerable slowdown in writing autodoc pages with sphinx 1.6.x sphinx-doc/sphinx#4230)
• ...

[jorisvandenbossche]

I think most discussion is in #18147 ?

[TomAugspurger]

Ah, indeed. Maybe that's why I didn't make a new issue earlier :)