Add isort pre-commit hook, sort imports in src and test directories

tests/tree/test_tree_get_node_by_id.py

@@ -1,5 +1,7 @@
-import pytest
 from typing import cast
+
+import pytest
+
 from textual.widgets import Tree
 from textual.widgets._tree import NodeID
 

tests/tree/test_tree_messages.py

@@ -1,9 +1,10 @@
 from __future__ import annotations
 
 from typing import Any
+
 from textual.app import App, ComposeResult
-from textual.widgets import Tree
 from textual.message import Message
+from textual.widgets import Tree
 
 
 class MyTree(Tree[None]):

tests/tree/test_tree_node_children.py

@@ -1,4 +1,5 @@
 import pytest
+
 from textual.widgets import Tree
 from textual.widgets.tree import TreeNode
 

tests/tree/test_tree_node_label.py

@@ -1,6 +1,7 @@
+from rich.text import Text
+
 from textual.widgets import Tree
 from textual.widgets.tree import TreeNode
-from rich.text import Text
 
 
 def test_tree_node_label() -> None: