dev-python/networkx: fix sphinx error

Bug: https://bugs.gentoo.org/594626
Package-Manager: Portage-2.3.6_p1, Repoman-2.3.2_p69

dev-python/networkx/files/1.11-sphinx-pngmath.patch

@@ -0,0 +1,14 @@
+Drop pngmath extenstion from sphinx config
+
+Bug: https://bugs.gentoo.org/594626
+
+--- a/doc/source/conf.py
++++ b/doc/source/conf.py
+@@ -74,7 +74,6 @@ def cd(newpath):
+     'sphinx.ext.intersphinx',
+     'sphinx.ext.mathjax',
+     'sphinx.ext.napoleon',
+-    'sphinx.ext.pngmath',
+     'sphinx.ext.todo',
+     'sphinx.ext.viewcode',
+     #'sphinxcontrib.bibtex',