403
in the response usually means you are trying to access an endpoint with either non-existing key or wrong type of key.
In this case - you cannot create classes using instance api key - using your account key is necessary. If you don't have it, you can always check it in the dashboard, or just use https://api.syncano.io/v1/account/auth/ endpoint to log in (or use api explorer: http://docs.syncano.com/v0.1/docs/account-log-in, account key will be in the response, once you pass your credentials).
Once you have your account key, you should use it to connect to your syncano instance:
import syncano
from syncano.models.base import Class
connection = syncano.connect(api_key="ACCOUNT_KEY")
book_class = Class.please.create(
instance_name='INSTANCE_NAME',
name='books',
schema=[
{"name": "title", "type": "string"},
{"name": "author", "type": "string"}
])
If account key and instance name are correct this code should work fine.