Fix .explain() in Django >=4.0

[SupImDos]

It looks like there was a change in Django 4.0 that broke the django-cte explain functionality.

See:

• django/django@fc91ea1
• https://code.djangoproject.com/ticket/27624#comment:12

This wasn't picked up by the unit tests because we manually set explain_query = True (which is now unused) rather than call the .explain() queryset method.

This PR adds some logic to retrieve explain_query for Django < 4.0, and explain_info for Django >= 4.0. It also updates the unit tests to call .explain() (which appears to work on sqlite3 as far as I can tell).

There's probably a cleaner way to do this, so feel free rework if you see fit.

[millerdev]

Thank you for the contribution!