Pass reason to @skip decorator

The decorator works without an argument as well, but that is an
undocumented feature of `unittest.skip()` that is not understood
by mypy and pytest.

In the case of pytest, it ignored the decorated class during test
collection, instead of collecting it and marking it as skipped.

tests/test_fields/test_field_tracker.py

@@ -870,7 +870,7 @@ class InheritedModelTrackerTests(ModelTrackerTests):
         self.assertTrue(self.tracker.has_changed('name2'))
 
 
-@skip
+@skip("has known failures")
 class AbstractModelTrackerTests(ModelTrackerTests):
 
     tracked_class = TrackedAbstract