python.pkgs.docutils: set LANG in case of darwin and python3

This should allow the Python3 version to be built while preventing mass-rebuilds.

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

@@ -1,4 +1,5 @@
-{ lib
+{ stdenv
+, lib
 , fetchurl
 , buildPythonPackage
 , isPy3k
@@ -14,11 +15,13 @@ buildPythonPackage rec {
     sha256 = "0x22fs3pdmr42kvz6c654756wja305qv6cx1zbhwlagvxgr4xrji";
   };
 
-  checkPhase = if isPy3k then ''
+  # Only Darwin needs LANG, but we could set it in general.
+  # It's done here conditionally to prevent mass-rebuilds.
+  checkPhase = lib.optionalString (isPy3k && stdenv.isDarwin) ''LANG="en_US.UTF-8" '' + (if isPy3k then ''
     ${python.interpreter} test3/alltests.py
   '' else ''
     ${python.interpreter} test/alltests.py
-  '';
+  '');
 
   # Create symlinks lacking a ".py" suffix, many programs depend on these names
   postFixup = ''