sage: ignore attrs deprecation warning

cmp is deprecated since attrs 19.2.0:
http://www.attrs.org/en/19.2.0/changelog.html

The deprecation warning breaks the doctests.  Fortunately they have a
rather long deprecation window, so we can just wait until upstream(s)
fix this.

pkgs/applications/science/math/sage/patches/ignore-cmp-deprecation.patch

@@ -0,0 +1,13 @@
+diff --git a/src/sage/tests/cmdline.py b/src/sage/tests/cmdline.py
+index bd6b76ab82..f8340a8c66 100644
+--- a/src/sage/tests/cmdline.py
++++ b/src/sage/tests/cmdline.py
+@@ -872,7 +872,7 @@ def test_executable(args, input="", timeout=100.0, **kwds):
+         sage: with open(input, 'w') as F:
+         ....:     _ = F.write(s)
+         sage: test_executable(["sage", "--rst2sws", input, output]) # py2
+-        ('', '', 0)
++        ('', '...', 0)
+         sage: import tarfile # py2
+         sage: f = tarfile.open(output, 'r') # py2
+         sage: print(f.extractfile('sage_worksheet/worksheet.html').read()) # py2

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

@@ -104,6 +104,9 @@ stdenv.mkDerivation rec {
       url = "https://git.sagemath.org/sage.git/patch?id=d27dc479a5772d59e4bc85d805b6ffd595284f1d";
       sha256 = "1nf1s9y7n30lhlbdnam7sghgaq9nasmv96415gl5jlcf7a3hlxk3";
     })
+
+    # ignore a deprecation warning for usage of `cmp` in the attrs library in the doctests
+    ./patches/ignore-cmp-deprecation.patch
   ];
 
   patches = nixPatches ++ bugfixPatches ++ packageUpgradePatches;