Updated ipytree warning for jlab3 (#721)

Jupyterlab 3 changed how widgets are installed. Now both the
frontend and backend are installed via pip/conda. If a user
also then installs the front using the `jupyter labextension install`
command then they can end up breaking things by trying to install
an old version.

Co-authored-by: jakirkham <[email protected]>

Author: ianhi

zarr/tests/test_util.py

@@ -179,7 +179,7 @@ def test_tree_widget_missing_ipytree():
     pattern = (
         "Run `pip install zarr[jupyter]` or `conda install ipytree`"
         "to get the required ipytree dependency for displaying the tree "
-        "widget. If using jupyterlab, you also need to run "
+        "widget. If using jupyterlab<3, you also need to run "
         "`jupyter labextension install ipytree`"
         )
     with pytest.raises(ImportError, match=re.escape(pattern)):

zarr/util.py

@@ -449,7 +449,7 @@ def tree_widget(group, expand, level):
         raise ImportError(
             "{}: Run `pip install zarr[jupyter]` or `conda install ipytree`"
             "to get the required ipytree dependency for displaying the tree "
-            "widget. If using jupyterlab, you also need to run "
+            "widget. If using jupyterlab<3, you also need to run "
             "`jupyter labextension install ipytree`".format(error)
         )