sage: support adding extra Python packages to environment

mysage = sage.override { extraPythonPackages = ps: with ps; [ dask ]; };

Note that the sage-tests are executed again as well, which is not very nice.

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

@@ -1,5 +1,6 @@
 { pkgs
 , withDoc ? false
+, extraPythonPackages ? ps: []
 }:
 
 # Here sage and its dependencies are put together. Some dependencies may be pinned
@@ -109,7 +110,7 @@ let
     rpy2
     sphinx
     pillow
-  ];
+  ] ++ extraPythonPackages python3.pkgs;
 
   pythonEnv = python3.buildEnv.override {
     extraLibs = pythonRuntimeDeps;