remove copy of sphinx inheritance_diagram.py

sphinx includes it since 0.6 (Mar 2009), as its only needed to build
documentation it should be safe to require this version.
Merge pull request #3249 from juliantaylor/remove-sphinx-inh-diag

remove copy of sphinx inheritance_diagram.py

sphinx includes it since 0.6 (Mar 2009), as its only needed to build
documentation it should be safe to require this version.