DeprecationWarning: The default pickle serializer is deprecated as of Pyramid 1.9 and it will be changed to use pyramid.session.JSONSerializer in version 2.0. Explicitly set the serializer to avoid future incompatibilities
. See "Upcoming Changes to ISession in Pyramid 2.0" for more information about this change.
As describe here https://docs.pylonsproject.org/projects/pyramid/en/1.10-branch/narr/sessions.html#pickle-session-deprecation
use a serializer that fallback on pickle if needed to avoid impacting users.