‘unirest’ in the documentation page does not correspond to python 3.
As usual you can easily use ‘requests’.
It should be written as follows.

import requests
from PIL import Image
from io import BytesIO

word = 'samurai'
mashapekey = '************* your mashape key *******************'

response= requests.get(
        'https://sunocos-kanjipicgenerator-v1.p.mashape.com/wahuKanji/'+word, 
        headers={'X-Mashape-Key': mashapekey})
img = Image.open(BytesIO(response.content))

img.show()
img.save('img.jpg', 'JPEG', quality=90, optimize=True)

You can see it without problems with firefox.
Also, if you press the download button and save it, you can see the image with safari.