Remove sphinx.ext.pngmath

Apparently this isn't found any more.

Author: aspiers

docs/conf.py

@@ -45,7 +45,7 @@
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = ['sphinx.ext.autodoc', 'sphinx.ext.intersphinx', 'sphinx.ext.todo',
               'sphinx.ext.autosummary', 'sphinx.ext.viewcode', 'sphinx.ext.coverage',
-              'sphinx.ext.doctest', 'sphinx.ext.ifconfig', 'sphinx.ext.pngmath',
+              'sphinx.ext.doctest', 'sphinx.ext.ifconfig',
               'sphinx.ext.napoleon']
 
 # Add any paths that contain templates here, relative to this directory.