Introduce the _HashObject protocol (#13553)

This protocol corresponds to what is called "hash object" in the hashlib documentation. In particular, it includes the non-OpenSSL BLAKE2 implementations which do not inherit HASH.

stdlib/_hashlib.pyi

@@ -2,13 +2,26 @@ import sys
 from _typeshed import ReadableBuffer
 from collections.abc import Callable
 from types import ModuleType
-from typing import AnyStr, final, overload
+from typing import AnyStr, Protocol, final, overload, type_check_only
 from typing_extensions import Self, TypeAlias
 
-_DigestMod: TypeAlias = str | Callable[[], HASH] | ModuleType | None
+_DigestMod: TypeAlias = str | Callable[[], _HashObject] | ModuleType | None
 
 openssl_md_meth_names: frozenset[str]
 
+@type_check_only
+class _HashObject(Protocol):
+    @property
+    def digest_size(self) -> int: ...
+    @property
+    def block_size(self) -> int: ...
+    @property
+    def name(self) -> str: ...
+    def copy(self) -> Self: ...
+    def digest(self) -> bytes: ...
+    def hexdigest(self) -> str: ...
+    def update(self, obj: ReadableBuffer, /) -> None: ...
+
 class HASH:
     @property
     def digest_size(self) -> int: ...