Tie intersphinx inventory locations to versions being installed

It may be rather useful is we would tie the location of the intersphinx inventories to the version of the packages we build the docs with. E.g. currently the docs build for astropy core is failing due to some unresolvable links, but the docs otherwise builds perfectly: https://github.com/astropy/astropy/issues/6695

The failures are due to some issues with the intersphinx of the brand new 2.1 of matplotlib. While for the docs we still use the 2.0.2 version. I foresee that similar issues could emerge later, too so it would be nice to link to actually resolve the links to the version we build the docs with.

Start by locating the intersphinx inventory configuration used by the documentation build and compare it with the package versions installed for the docs. Check how matplotlib 2.0.2 and other dependency versions are represented in the inventory URLs. Done means links resolve against inventories matching the versions used to build the documentation.

Written by the indexing model from the issue text.