I measured various implementations of __instancecheck__ using
`./python.exe -m timeit -s 'from rational import Rational; r =
Rational(3, 2)' '...'` on my 2.33 GHz MacBook, with ... replaced by
either isinstance(r, Rational) or isinstance(3, Rational) to measure
both the positive and negative cases. The big win comes from avoiding
the genexp and the set. Then we win smaller amounts by being more
careful about avoiding extra calls to __subclasscheck__ and by inlining
the cache checks.

# Current code
    return any(cls.__subclasscheck__(c)
               for c in set([instance.__class__, type(instance)]))
isinstance(3, Rational): 4.65 usec
isinstance(r, Rational): 7.47 usec

# The best we can do simply in Python
    return cls.__subclasscheck__(instance.__class__)
isinstance(3, Rational): 2.08 usec
isinstance(r, Rational): 1.72 usec

# Preserve behavior, simply
    return (cls.__subclasscheck__(instance.__class__) or
            cls.__subclasscheck__(type(instance)))
isinstance(3, Rational): 3.03 usec
isinstance(r, Rational): 1.8 usec

# Preserve behavior, complexly
    ic = instance.__class__
    if cls.__subclasscheck__(ic):
        return True
    t = type(instance)
    return t is not ic and cls.__subclasscheck__(t)
isinstance(3, Rational): 2.38 usec
isinstance(r, Rational): 1.86 usec

# Inlined for new-style classes
    subclass = instance.__class__
    if subclass in cls._abc_cache:
        return True
    type_ = type(instance)
    if type_ is subclass:
        if (cls._abc_negative_cache_version ==
            ABCMeta._abc_invalidation_counter and
            subclass in cls._abc_negative_cache):
            return False
        return cls.__subclasscheck__(subclass)
    return (cls.__subclasscheck__(subclass) or
            cls.__subclasscheck__(type_))
isinstance(3, Rational): 2.26 usec
isinstance(r, Rational): 1.49 usec