python3Packages.python-crontab: skip test test_03_usage

to my knowledge this test only fails on darwin, but assuming /tmp
is writable is a Bad Thing anyway so skipping on all platforms.
don't think it covers anything not otherwise covered.

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

@@ -10,7 +10,11 @@ buildPythonPackage rec {
   };
 
   checkInputs = [ pytestCheckHook ];
-  disabledTests = [ "test_07_non_posix_shell"];
+  disabledTests = [
+    "test_07_non_posix_shell"
+    # doctest that assumes /tmp is writeable, awkward to patch
+    "test_03_usage"
+  ];
 
   propagatedBuildInputs = [ python-dateutil ];