Fix constructor signatures to 2.7/cPickle.{Pickler,Unpickler}.

stdlib/2.7/cPickle.pyi

@@ -4,8 +4,12 @@ HIGHEST_PROTOCOL = ...  # type: int
 compatible_formats = ...  # type: List[str]
 format_version = ...  # type: str
 
-class Pickler: ...
-class Unpickler: ...
+class Pickler:
+    def __init__(self, file: IO[str], protocol: int = ...) -> None: ...
+
+
+class Unpickler:
+    def __init__(self, file: IO[str]) -> None: ...
 
 def dump(obj: Any, file: IO[str], protocol: int = ...) -> None: ...
 def dumps(obj: Any, protocol: int = ...) -> str: ...