python/qrcode: Add mock to checkInputs

Another test issue surfacing from the setuptools bump in
70c1899768.

This time, the tests just fail because mock is missing as a dependency.

Tested building against Python 2.7 and Python 3.6.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @joachifm, @Mic92

pkgs/top-level/python-packages.nix

@@ -19517,6 +19517,7 @@ in {
     };
 
     propagatedBuildInputs = with self; [ six pillow pymaging_png ];
+    checkInputs = [ self.mock ];
 
     meta = {
       description = "Quick Response code generation for Python";