sage: fix test suite with docutils 0.15

The better way to fix this would be to backport the upstream sphinx
patch:

faedcc48cc

Unfortunately it doesn't apply cleanly and isn't worth the effort
of backporting. Let's hope we can switch to python3 sage and the recent
sphinx version that comes with it before this becomes a problem.

pkgs/applications/science/math/sage/patches/docutils-0.15.patch

@@ -0,0 +1,24 @@
+diff --git a/src/sage/misc/sphinxify.py b/src/sage/misc/sphinxify.py
+index 4849c2bffa..76b7bc8602 100644
+--- a/src/sage/misc/sphinxify.py
++++ b/src/sage/misc/sphinxify.py
+@@ -25,6 +25,7 @@ from __future__ import absolute_import, print_function
+ import os
+ import re
+ import shutil
++import warnings
+ from tempfile import mkdtemp
+ from sphinx.application import Sphinx
+ 
+@@ -120,7 +121,10 @@ smart_quotes = no""")
+     # buildername, confoverrides, status, warning, freshenv).
+     sphinx_app = Sphinx(srcdir, confdir, outdir, doctreedir, format,
+                         confoverrides, None, None, True)
+-    sphinx_app.build(None, [rst_name])
++    with warnings.catch_warnings():
++        # Quick and dirty workaround for https://trac.sagemath.org/ticket/28856#comment:19
++        warnings.simplefilter("ignore")
++        sphinx_app.build(None, [rst_name])
+     sys.path = old_sys_path
+ 
+     # We need to remove "_" from __builtin__ that the gettext module installs

pkgs/applications/science/math/sage/sage-src.nix

@@ -52,6 +52,11 @@ stdenv.mkDerivation rec {
     # Parallelize docubuild using subprocesses, fixing an isolation issue. See
     # https://groups.google.com/forum/#!topic/sage-packaging/YGOm8tkADrE
     ./patches/sphinx-docbuild-subprocesses.patch
+
+    # Fix doctest failures with docutils 0.15:
+    # https://nix-cache.s3.amazonaws.com/log/dzmzrb2zvardsmpy7idg7djkizmkzdhs-sage-tests-8.9.drv
+    # https://trac.sagemath.org/ticket/28856#comment:19
+    ./patches/docutils-0.15.patch
   ];
 
   # Since sage unfortunately does not release bugfix releases, packagers must