Use TypeAlias where possible for type aliases (#7630)

stdlib/urllib/parse.pyi

@@ -1,5 +1,6 @@
 import sys
 from typing import Any, AnyStr, Callable, Generic, Mapping, NamedTuple, Sequence, overload
+from typing_extensions import TypeAlias
 
 if sys.version_info >= (3, 9):
     from types import GenericAlias
@@ -28,7 +29,7 @@ __all__ = [
     "SplitResultBytes",
 ]
 
-_Str = bytes | str
+_Str: TypeAlias = bytes | str
 
 uses_relative: list[str]
 uses_netloc: list[str]