Pickling of objects of classes whose __new__ mandates the use of keyword-only arguments is supported with protocol 4 (using a new opcode NEWOBJ_EX). But it is possible to implement this feature with protocol 2+ (less efficiently than with NEWOBJ_EX). __new_ex__ is pickled as partial(cls.__new__, cls, *args, **kwargs). Pickled data is compatible with older Python releases up to 2.7 (issue5228).

Proposed patch adds support of __new__ with keyword arguments with protocols 2 and 3.