feat(core): include Django tests.py convention in Python group

Django's per-app single-file test module — historically the tutorial's
default before pytest-django popularised the tests/ package layout — is
still ubiquitous in Django and Django-derived projects (e.g. odoo,
django/contrib/admin/tests.py).

Add **/tests.py so users who opt-in also catch that convention. Kept
distinct from **/test_*.py because it's a framework idiom, not a pytest
discovery rule.