Use PEP 604 syntax wherever possible (#7493)

stdlib/lib2to3/pytree.pyi

@@ -1,8 +1,8 @@
 from _typeshed import Self
 from lib2to3.pgen2.grammar import Grammar
-from typing import Any, Callable, Iterator, Optional, Union
+from typing import Any, Callable, Iterator, Optional
 
-_NL = Union[Node, Leaf]
+_NL = Node | Leaf
 _Context = tuple[str, int, int]
 _Results = dict[str, _NL]
 _RawNode = tuple[int, str, _Context, Optional[list[_NL]]]