Use TypeAlias where possible for type aliases (#7630)

stdlib/pickle.pyi

@@ -1,6 +1,6 @@
 import sys
 from typing import Any, Callable, ClassVar, Iterable, Iterator, Mapping, Protocol, Union
-from typing_extensions import final
+from typing_extensions import TypeAlias, final
 
 if sys.version_info >= (3, 8):
     __all__ = [
@@ -189,7 +189,7 @@ if sys.version_info >= (3, 8):
         def __init__(self, buffer: Any) -> None: ...
         def raw(self) -> memoryview: ...
         def release(self) -> None: ...
-    _BufferCallback = Callable[[PickleBuffer], Any] | None
+    _BufferCallback: TypeAlias = Callable[[PickleBuffer], Any] | None
     def dump(
         obj: Any,
         file: _WritableFileobj,
@@ -223,7 +223,7 @@ class PickleError(Exception): ...
 class PicklingError(PickleError): ...
 class UnpicklingError(PickleError): ...
 
-_reducedtype = Union[
+_reducedtype: TypeAlias = Union[
     str,
     tuple[Callable[..., Any], tuple[Any, ...]],
     tuple[Callable[..., Any], tuple[Any, ...], Any],