Defining complex.__complex__ and bytes.__bytes__ would not solve anything, because

>>> issubclass(int, SupportsComplex)
False
>>> issubclass(float, SupportsComplex)
False
>>> issubclass(bytearray, SupportsBytes)
False
>>> issubclass(memoryview, SupportsBytes)
False

If SupportsComplex and SupportsBytes are just for "has __complex__/__bytes__ method", they are virtually useless. If their meaning is "can be converted to complex/bytes", it is different story, and it should be fixed be adding subclasshooks which check existence of alternate methods (__float__, __index__, supporting the buffer protocol).