Add mock to the list of whisper's test inputs.

This was required as the versiof whisper was upgraded from
1.1.3 to 1.1.4 and mock was added as a test dependency in
that release:
https://github.com/graphite-project/whisper/commit/703dd9d3a60a0f79383c03a541ee66bd50966a01#diff-62849af500c571aa4532da7be8833e73R14

pkgs/development/python-modules/whisper/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, buildPythonPackage, fetchPypi, six }:
+{ stdenv, buildPythonPackage, fetchPypi, mock, six }:
 
 buildPythonPackage rec {
   pname = "whisper";
@@ -10,6 +10,7 @@ buildPythonPackage rec {
   };
 
   propagatedBuildInputs = [ six ];
+  checkInputs           = [ mock ];
 
   meta = with stdenv.lib; {
     homepage = http://graphite.wikidot.com/;