Stop bundling the numpydoc Sphinx extension

numpydoc is now a separately installable package, so we can express a
dependency for the 'doc' install option, rather than bundling it. This
gets us bugfixes that our copy was lacking - one of which was causing a
problem for the ShiningPanda docs job.
Bump Sphinx version dependency
Merge pull request #4670 from takluyver/no-bundle-numpydoc

Stop bundling the numpydoc Sphinx extension

numpydoc is now a separately installable package, so we can express a dependency for the 'doc' install option, rather than bundling it. This gets us bugfixes that our copy was lacking - one of which was causing a problem for the ShiningPanda docs job.