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__instancecheck__ contains unnecessary code. If we restrict ABCs to new
style classes we could make the function faster:
Old:
 def __instancecheck__(cls, instance):
 """Override for isinstance(instance, cls)."""
 return any(cls.__subclasscheck__(c)
 for c in {instance.__class__, type(instance)})
New:
 def __instancecheck__(cls, instance):
 """Override for isinstance(instance, cls)."""
 # safe a function call for common case
 if instance.__class__ in cls._abc_cache:
 return True
 return cls.__subclasscheck__(instance.__class__)