python.pkgs.matplotlib: add numpy 1.6 compat patch

Numpy deprecated asscalar in 1.16. This will be fixed in the next
matplotlib version, but to avoid deprecation warnings (and because they
are breaking the sage testsuite), it can't hurt to backport the fix
already.

Upstream: https://github.com/matplotlib/matplotlib/pull/12478

pkgs/development/python-modules/matplotlib/2.nix

@@ -1,6 +1,7 @@
 { stdenv, fetchPypi, python, buildPythonPackage, pycairo, backports_functools_lru_cache
 , which, cycler, dateutil, nose, numpy, pyparsing, sphinx, tornado, kiwisolver
 , freetype, libpng, pkgconfig, mock, pytz, pygobject3, functools32, subprocess32
+, fetchpatch
 , enableGhostscript ? false, ghostscript ? null, gtk3
 , enableGtk2 ? false, pygtk ? null, gobject-introspection
 , enableGtk3 ? false, cairo
@@ -47,9 +48,16 @@ buildPythonPackage rec {
     ++ stdenv.lib.optionals enableQt [ pyqt4 ]
     ++ stdenv.lib.optionals python.isPy2 [ functools32 subprocess32 ];
 
-  patches =
-    [ ./basedirlist.patch ] ++
-    stdenv.lib.optionals stdenv.isDarwin [ ./darwin-stdenv-2.2.3.patch ];
+  patches = [
+    ./basedirlist.patch
+
+    # https://github.com/matplotlib/matplotlib/pull/12478
+    (fetchpatch {
+      name = "numpy-1.16-compat.patch";
+      url = "https://github.com/matplotlib/matplotlib/commit/2980184d092382a40ab21f95b79582ffae6e19d6.patch";
+      sha256 = "1c0wj28zy8s5h6qiavx9zzbhlmhjwpzbc3fyyw9039mbnqk0spg2";
+    })
+  ] ++ stdenv.lib.optionals stdenv.isDarwin [ ./darwin-stdenv-2.2.3.patch ];
 
   # Matplotlib tries to find Tcl/Tk by opening a Tk window and asking the
   # corresponding interpreter object for its library paths. This fails if