Django unsupported pickle protocol 4 after downgrading Python version from 3 to 2

After downgrading Python version used by a Django app that I'm working on, I started getting errors about deserialization, mainly affecting session & cache management.

ValueError at  
unsupported pickle protocol: 4  

This was due to my database (Postgres) & cache store (Redis) containing values that were serialized using more recent pickle version than the one supported in Python 2.

So the simplest solution was to delete existing sessions & clear cache:

from django.contrib.sessions.models import Session  
Session.objects.all().delete()  

from django.core.cache import cache  
cache.clear()