MAINT/DOC: pngmath deprecated for sphinx >= 1.4 (scikit-learn#7279)

Fix scikit-learn#7222

Author: GaelVaroquaux

doc/conf.py

@@ -32,11 +32,20 @@
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
     'sphinx.ext.autodoc', 'sphinx.ext.autosummary',
-    'sphinx.ext.pngmath', 'numpy_ext.numpydoc',
+    'numpy_ext.numpydoc',
     'sphinx.ext.linkcode', 'sphinx.ext.doctest',
     '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')
+
+
 autodoc_default_flags = ['members', 'inherited-members']
 
 # Add any paths that contain templates here, relative to this directory.