dbm: fix bytes handling (#9030)

stdlib/dbm/dumb.pyi

@@ -10,6 +10,9 @@ _ValueType: TypeAlias = str | bytes
 
 error = OSError
 
+# This class doesn't exist at runtime. open() can return an instance of
+# any of the three implementations of dbm (dumb, gnu, ndbm), and this
+# class is intended to represent the common interface supported by all three.
 class _Database(MutableMapping[_KeyType, bytes]):
     def __init__(self, filebasename: str, mode: str, flag: str = ...) -> None: ...
     def sync(self) -> None: ...