Fix urllib3 DeprecationWarning in client.py

>>> from nuxeo.client import Nuxeo
>>> nuxeo = Nuxeo(host="http://192.168.2.39:8080/nuxeo/")
.../urllib3/util/retry.py:252: DeprecationWarning: Using 'method_whitelist' with Retry is deprecated and will be removed in v2.0. Use 'allowed_methods' instead