🚨(backend) follow Ruff 2024.2 style introduced in v0.3.0

We recently updated Ruff from 0.2.2 to v0.3, which introduced
Ruff 2024.2 style. This new style updated Ruff formatter's behavior,
making our make lint command fails.

Ruff 2024.2 style add a blank line after the module docstring.
Please take a look at Ruff ChangeLog to get more info.

src/backend/core/admin.py

@@ -1,4 +1,5 @@
 """Admin classes and registrations for People's core app."""
+
 from django import forms
 from django.contrib import admin
 from django.contrib.auth import admin as auth_admin