[Reminder]Use JSONField to store qualifiers in django package mixin

[sbs2001]

Qualifiers are currently stored as strings in the database, which implies that the order of each qualifier matters. This contradicts the Package URL specification ( check for more details https://github.com/package-url/purl-spec/issues/51#issuecomment-455563672 )

https://github.com/django/django/pull/12392 adds JSONField in a db agnostic way to the django orm.

This field will be implemented for Django 3.1+ is not planned to be backported.