Make default_app_config conditional

[nickmoreton]

Description

This adds a conditional around default_app_config in init.py

The result is when using Django >= 3.2 there are no longer deprecation warnings shown in the console. The warnings can be enabled with python -Wa manage.py check

Fixes # (159)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[thibaudcolas]

Thank you @nickmoreton, looks great to me. I’ll just change the CHANGELOG entry to mention default_app_config as part of releasing.