Does a bike exist that is both very light and does not need to be ridden in a hunched over position? I have a spinal injury that prevents me from lifting anything heavier than about 10 lbs (4.5 kg), and prevents me from being in a hunched over position, without causing myself extreme and lingering (for weeks or months) pain. I miss biking. I am wondering if there exists a bike that is both very light, but also allows for riding in an upright sitting position? So far my searches have turned up nothing.

It's possible I just have to resign myself to not being able to bike anymore, but I am hoping that there is some niche build or light bike that could be modified out there that might be suitable that I have missed in my research so far. 

EDIT: the bike does not need to be less than 10 lbs (4.5 kg). I just wanted to give perspective on the necessity for low weight. I weigh about 170 lbs (77 kg)
Here's what you need!  Lightweight and upright --