fix mypy and add pull number to whats-new.rst

Author: eshort0401

doc/whats-new.rst

@@ -29,7 +29,7 @@ Bug Fixes
 - Ensure that ``keep_attrs='drop'`` and ``keep_attrs=False`` remove attrs from result, even when there is
   only one xarray object given to ``apply_ufunc`` (:issue:`10982` :pull:`10997`).
   By `Julia Signell <https://github.com/jsignell>`_.
-- Forbid child names containing ``/`` in ``DataTree`` objects (:issue:`#9978` :issue:`#9490`).
+- Forbid child names containing ``/`` in ``DataTree`` objects (:issue:`#9978` :issue:`#9490` :pull:`#11014`).
   By `Ewan Short <https://github.com/eshort0401>`_.
 
 Documentation

xarray/tests/test_datatree.py

@@ -149,7 +149,7 @@ def test_child_containing_slashes(self) -> None:
         with pytest.raises(ValueError, match=message):
             dt: DataTree = DataTree(dataset=ds, children={"x/y": DataTree()})
         with pytest.raises(ValueError, match=message):
-            dt: DataTree = DataTree(dataset=ds, children={"x/y": DataTree()})
+            dt = DataTree(dataset=ds, children={"x/y": DataTree()})
             dt.children = {"x/y": DataTree()}