"401 Unauthorized" and "403 Forbidden" are two different HTTP errors.
CouchDB uses the first one when the server requires authentication
credentials, and the second one when the request requires an
authorisation that the current user does not have.

For instance, using a `validate_doc_update` design document can result
in "403 Forbidden" being returned. See the official documentation [1]
for more details and examples:

  // user is not authorized to make the change but may re-authenticate
  throw({ unauthorized: 'Error message here.' });
  // change is not allowed
  throw({ forbidden: 'Error message here.' });

This patch adds Forbidden to the list of HTTP errors raised by
couchdb-python.

[1]: http://docs.couchdb.org/en/stable/couchapp/ddocs.html#validate-document-update-functions