ANTIALIAS has been removed from the latest Python module pillow, and hence, your code is giving the error. You can use "LANCZOS" instead of "ANTIALIAS" to fix the error.
Here is the modified code that will work:
from PIL import Image
inImg = Image.open("example.jpg")
size = (100, 100)
inImg.thumbnail(size, Image.LANCZOS)
If you want to use ANTIALIAS, you need to use an older (< 10.0.0) version of "pillow".