Description: remove unused and obsolete pngmath sphinx extension
Bug-Debian: https://bugs.debian.org/918816
Author: Dmitry Shachnev <mitya57@debian.org>

--- a/docs/conf.py
+++ b/docs/conf.py
@@ -25,7 +25,7 @@
 
 # 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.autodoc', 'sphinx.ext.coverage', 'sphinx.ext.pngmath', 'sphinx.ext.ifconfig']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.coverage', 'sphinx.ext.ifconfig']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
