docs: Support pngmath replacement (sphinx >= 1.8)

The pngmath module was removed in favor of the imgmath module in 1.8
Thanks to avsej for drawing attention to this.

Author: jplloyd

doc/source/conf.py

@@ -27,7 +27,13 @@
 
 # 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.doctest', 'sphinx.ext.intersphinx', 'sphinx.ext.todo', 'sphinx.ext.coverage', 'sphinx.ext.pngmath', 'sphinx.ext.ifconfig', 'sphinx.ext.viewcode']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest', 'sphinx.ext.intersphinx', 'sphinx.ext.todo', 'sphinx.ext.coverage', 'sphinx.ext.ifconfig', 'sphinx.ext.viewcode']
+
+try:
+    import sphinx.ext.pngmath
+    extensions.append('sphinx.ext.pngmath')
+except ImportError:
+    extensions.append('sphinx.ext.imgmath')
 
 # Breathe setup, for integrating doxygen content
 extensions.append('breathe')