Run black over recent Tree tests

Now that we're running black on tests...

tests/tree/test_tree_node_children.py

@@ -1,6 +1,7 @@
 import pytest
 from textual.widgets import Tree, TreeNode
 
+
 def label_of(node: TreeNode[None]):
     """Get the label of a node as a string"""
     return str(node.label)
@@ -8,17 +9,21 @@ def label_of(node: TreeNode[None]):
 
 def test_tree_node_children() -> None:
     """A node's children property should act like an immutable list."""
-    CHILDREN=23
+    CHILDREN = 23
     tree = Tree[None]("Root")
     for child in range(CHILDREN):
         tree.root.add(str(child))
-    assert len(tree.root.children)==CHILDREN
+    assert len(tree.root.children) == CHILDREN
     for child in range(CHILDREN):
         assert label_of(tree.root.children[child]) == str(child)
     assert label_of(tree.root.children[0]) == "0"
-    assert label_of(tree.root.children[-1]) == str(CHILDREN-1)
-    assert [label_of(node) for node in tree.root.children] == [str(n) for n in range(CHILDREN)]
-    assert [label_of(node) for node in tree.root.children[:2]] == [str(n) for n in range(2)]
+    assert label_of(tree.root.children[-1]) == str(CHILDREN - 1)
+    assert [label_of(node) for node in tree.root.children] == [
+        str(n) for n in range(CHILDREN)
+    ]
+    assert [label_of(node) for node in tree.root.children[:2]] == [
+        str(n) for n in range(2)
+    ]
     with pytest.raises(TypeError):
         tree.root.children[0] = tree.root.children[1]
     with pytest.raises(TypeError):