Make your background RGB, not RGBA. And remove the later conversion of the background to RGB, of course, since it’s already in that mode. This worked for me with a test image I created:
from PIL import Image
im = Image.open(r"C:\jk.png")
bg = Image.new("RGB", im.size, (255,255,255))
bg.paste(im,im)
bg.save(r"C:\jk2.jpg")