tests/examples/_internal/frozen_dataclass_sealable/test_basic.py(style): Update imports and formatting

why: Ensure consistency with project coding standards.

what:
- Added __future__ import for annotations
- Reorganized imports with standard library first
- Fixed whitespace and line breaks
- Simplified type annotations using PEP 604 syntax (Union | None)
- Improved code organization

refs: Maintains code quality in example test files

Author: tony

tests/examples/_internal/frozen_dataclass_sealable/test_basic.py

@@ -6,10 +6,11 @@
 working code examples.
 """
 
-import pytest
+from __future__ import annotations
+
+from dataclasses import field
 
-from dataclasses import dataclass, field
-from typing import Optional
+import pytest
 
 from libtmux._internal.frozen_dataclass_sealable import (
     frozen_dataclass_sealable,
@@ -19,72 +20,72 @@
 
 def test_basic_usage():
     """Test basic usage of frozen_dataclass_sealable."""
+
     @frozen_dataclass_sealable
     class Config:
         name: str
 
         values: dict[str, int] = field(
-            default_factory=dict,
-            metadata={"mutable_during_init": True}
+            default_factory=dict, metadata={"mutable_during_init": True}
         )
 
     # Create an instance
     config = Config(name="test-config")
     assert config.name == "test-config"
-    
+
     # Cannot modify immutable fields
     with pytest.raises(AttributeError):
         config.name = "modified"
-    
+
     # Can modify mutable fields
     config.values["key1"] = 100
     assert config.values["key1"] == 100
 
     # Check sealable property
     assert is_sealable(config)
-    
+
     # Seal the object
     config.seal()
     assert hasattr(config, "_sealed") and config._sealed
-    
+
     # Can still modify contents of mutable containers after sealing
     config.values["key2"] = 200
     assert config.values["key2"] == 200
 
 
 def test_deferred_sealing():
     """Test deferred sealing with linked nodes."""
+
     @frozen_dataclass_sealable
     class Node:
         value: int
 
-        next_node: Optional['Node'] = field(
-            default=None,
-            metadata={"mutable_during_init": True}
+        next_node: Node | None = field(
+            default=None, metadata={"mutable_during_init": True}
         )
 
     # Create a linked list (not circular to avoid recursion issues)
     node1 = Node(value=1)
     node2 = Node(value=2)
     node1.next_node = node2
-    
+
     # Verify structure
     assert node1.value == 1
     assert node2.value == 2
     assert node1.next_node is node2
-    
+
     # Verify sealable property
     assert is_sealable(node1)
     assert is_sealable(node2)
-    
-    # Seal nodes individually 
+
+    # Seal nodes individually
     node1.seal()
     node2.seal()
-    
+
     # Verify both nodes are sealed
     assert hasattr(node1, "_sealed") and node1._sealed
     assert hasattr(node2, "_sealed") and node2._sealed
-    
+
     # Verify immutability after sealing
     with pytest.raises(AttributeError):
         node1.value = 10