Replace sphinx.ext.pngmath with sphinx.ext.imgmath

pngmath is EOL sphinx-doc/sphinx#6739

Author: manufactory

doc/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.doctest', 'sphinx.ext.todo', 'sphinx.ext.coverage', 'sphinx.ext.pngmath', 'sphinx.ext.ifconfig', 'sphinx.ext.viewcode']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.doctest', 'sphinx.ext.todo', 'sphinx.ext.coverage', 'sphinx.ext.imgmath', 'sphinx.ext.ifconfig', 'sphinx.ext.viewcode']
 #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']
 
 # Add any paths that contain templates here, relative to this directory.
@@ -224,4 +224,3 @@
 autoclass_content = 'both'
 
 todo_include_todos = True
-