I have no doubt this will get into 3.x once we have a working patch and backward compatibility issues are addressed.  Given the amount of effort Victor has recently put in #22117 to rework CPython internal time handling to support nanoseconds, it will be odd not to support nanoseconds in datetime.

On the substance, in your patch you have chosen to add nanoseconds as a separate field instead of changing microseconds to nanoseconds.  I don't think this is the right approach.  See msg223082.  Once you get to implementing timedelta arithmetics, you will see that carrying two subsecond fields will unnecessarily complicate the code.