Removes old dependency

Author: kushaldas

docs/conf.py

@@ -29,7 +29,7 @@
 
 # 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.intersphinx', 'sphinx.ext.pngmath']
+extensions = ['sphinx.ext.intersphinx',]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']