DOC: Remove Sphinx extension for inheritence trees.

doc/source/conf.py

@@ -32,8 +32,7 @@ except:
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = ['sphinx.ext.autodoc', 'sphinx.ext.pngmath', 'numpydoc',
-              'sphinx.ext.autosummary', 'sphinx.ext.inheritance_diagram',
-              'plot_directive', 'gen_rst']
+              'sphinx.ext.autosummary', 'plot_directive', 'gen_rst']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']