sage: ignore cmp deprecation in sagenb

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

@@ -0,0 +1,13 @@
+diff --git a/sagenb/__init__.py b/sagenb/__init__.py
+index 4db0d2cb..2fc5f01e 100644
+--- a/sagenb/__init__.py
++++ b/sagenb/__init__.py
+@@ -1,3 +1,8 @@
+ # -*- coding: utf-8 -*
+   # init
++import warnings
+ from . import storage
++
++# deprecation in attrs, needs to be fixed in twisted
++warnings.filterwarnings('ignore', category=DeprecationWarning,
++    message=r'The usage of `cmp` is deprecated and will be removed.*')

pkgs/applications/science/math/sage/sagenb.nix

@@ -26,6 +26,11 @@ buildPythonPackage rec {
     sha256 = "0bxvhr03qh2nsjdfc4pyfiqrn9jhp3vf7irsc9gqx0185jlblbxs";
   };
 
+  patches = [
+    # cmp deprecation in attrs needs to be handled in twisted
+    ./patches/sagenb-cmp-deprecation.patch
+  ];
+
   propagatedBuildInputs = [
     twisted
     flask