use imgmath since pmgmath is deprecated

doc/conf.py

@@ -46,7 +46,7 @@ needs_sphinx = '1.3'
 extensions = [
     'sphinx.ext.todo',
     'sphinx.ext.coverage',
-    'sphinx.ext.pngmath',
+    'sphinx.ext.imgmath',
     'sphinx.ext.ifconfig',
     'sphinx.ext.autodoc',
     'sphinx.ext.autosummary',
@@ -69,7 +69,7 @@ autosummary_generate = True
 
 # If we are on OSX, the 'dvipng' path maybe different
 dvipng_osx = '/opt/local/libexec/texlive/binaries/dvipng'
-if os.path.exists(dvipng_osx): pngmath_dvipng = dvipng_osx
+if os.path.exists(dvipng_osx): imgmath_dvipng = dvipng_osx
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']