Support generic class (i.e. __class_getitem__) #2917

>>> class Generic:
...   def __class_getitem__(cls, item, *args, **kwargs):
...     return (cls, item, args, kwargs)
... 
>>> Generic[int, str]
(<class '__main__.Generic'>, (<class 'int'>, <class 'str'>), (), {})

>>>>> class Generic:
.....   def __class_getitem__(cls, item, *args, **kwargs):
.....     return (cls, item, args, kwargs)
..... 
>>>>> Generic[int, str]
<class 'type'>