Use TypeAlias where possible for type aliases (#7630)

stdlib/_codecs.pyi

@@ -1,13 +1,14 @@
 import codecs
 import sys
 from typing import Any, Callable
+from typing_extensions import TypeAlias
 
 # This type is not exposed; it is defined in unicodeobject.c
 class _EncodingMap:
     def size(self) -> int: ...
 
-_MapT = dict[int, int] | _EncodingMap
-_Handler = Callable[[Exception], tuple[str, int]]
+_MapT: TypeAlias = dict[int, int] | _EncodingMap
+_Handler: TypeAlias = Callable[[Exception], tuple[str, int]]
 
 def register(__search_function: Callable[[str], Any]) -> None: ...
 def register_error(__errors: str, __handler: _Handler) -> None: ...