sphinx-rtd-theme: fix missing docutils for cross compilation

The tests are disabled when cross compiling. The effect of adding
pythonRelaxDepsHook to check inputs is that it is skipped when cross
compiling or when checks are disabled.

pkgs/development/python-modules/sphinx-rtd-theme/default.nix

@@ -31,9 +31,12 @@ buildPythonPackage rec {
     sphinxcontrib-jquery
   ];
 
+  nativeBuildInputs = [
+    pythonRelaxDepsHook
+  ];
+
   nativeCheckInputs = [
     pytestCheckHook
-    pythonRelaxDepsHook
     readthedocs-sphinx-ext
   ];