base64: fix a85decode param type (#9014)

Author: JelleZijlstra

stdlib/base64.pyi

@@ -42,7 +42,9 @@ if sys.version_info >= (3, 10):
     def b32hexdecode(s: str | ReadableBuffer, casefold: bool = ...) -> bytes: ...
 
 def a85encode(b: ReadableBuffer, *, foldspaces: bool = ..., wrapcol: int = ..., pad: bool = ..., adobe: bool = ...) -> bytes: ...
-def a85decode(b: str | ReadableBuffer, *, foldspaces: bool = ..., adobe: bool = ..., ignorechars: str | bytes = ...) -> bytes: ...
+def a85decode(
+    b: str | ReadableBuffer, *, foldspaces: bool = ..., adobe: bool = ..., ignorechars: bytearray | bytes = ...
+) -> bytes: ...
 def b85encode(b: ReadableBuffer, pad: bool = ...) -> bytes: ...
 def b85decode(b: str | ReadableBuffer) -> bytes: ...
 def decode(input: IO[bytes], output: IO[bytes]) -> None: ...