MAINT/DOC: pngmath deprecated for sphinx >= 1.4

ported from scikit-learn/scikit-learn#7279

Author: TomDLT

doc/conf.py

@@ -43,13 +43,20 @@
     'sphinx.ext.intersphinx',
     'sphinx.ext.todo',
     'numpydoc',
-    'sphinx.ext.pngmath',
     'sphinx.ext.ifconfig',
     'sphinx.ext.viewcode',
     'sphinx_gallery.gen_gallery'
 
 ]
 
+# pngmath / imgmath compatibility layer for different sphinx versions
+import sphinx
+from distutils.version import LooseVersion
+if LooseVersion(sphinx.__version__) < LooseVersion('1.4'):
+    extensions.append('sphinx.ext.pngmath')
+else:
+    extensions.append('sphinx.ext.imgmath')
+
 sphinx_gallery_conf = {
     # path to your examples scripts
     'examples_dirs' : '../examples',