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.

docs/source/conf.py

@@ -49,7 +49,7 b' extensions = ['
     'matplotlib.sphinxext.plot_directive',
     'sphinx.ext.autodoc',
     'sphinx.ext.doctest',
-    'inheritance_diagram',
+    'sphinx.ext.inheritance_diagram',
     'ipython_console_highlighting',
     'ipython_directive',
     'numpydoc',  # to preprocess docstrings