Loading detection as it was originally using querry_selector(.stop).is_enabled() didn't work on my machine, perhaps bing chat website was changed and that button doesnt have the enabled attribute.
I tried to turn condition to is_visible(), but the button didnt have that attribute either.
During debugging this I found that button appears on typing indicator "panel" (cib_typing_indicator), and it changes itself when typing response, adds cancelable attribute to its element.
It is None when typing is not active (i.e before or after typing was completed) 
Seems to work more or less reliable on my machine.
The time.sleep(7) in get_last_message() probably should be changed to some computed value as it may depend on CPU/Network speed, but it was a quick hack.