Sync nearbyshops with the updated tutorial

[realpython-bot]

Syncs nearbyshops/ with the maintenance update of Make a Location-Based Web App With Django and GeoDjango.

What changed

• shops/admin.py: OSMGeoAdmin was removed in Django 5.0, so the admin now uses GISModelAdmin, matching the updated article.
• nearbyshops/settings.py: dropped USE_L10N (removed in Django 5.0) and set DEFAULT_AUTO_FIELD = "django.db.models.AutoField" to match the existing 0001_initial migration and silence models.W042.
• requirements.txt (new): pins the article's dependencies field, django==6.1.1 and psycopg2-binary==2.9.13.
• readme.md: adds a Requirements section (Python 3.12 or later, which is Django 6.1's floor, plus GDAL/GEOS/PROJ and a PostGIS database) and the kartoza/postgis:18-3.6 command the article now uses.

Verification

• Fresh venv on Python 3.14.6 with the new pins, against PostgreSQL 16 + PostGIS 3.4 (GDAL/GEOS/PROJ from Ubuntu 24.04).
• manage.py check: no issues. manage.py migrate applies both migrations and loads all 100 shops from data.json.
• The test client returned 200 for / (the six nearest shops) and for the admin changelist, add, and change pages. The add and change pages render the OSM map widget. runserver serves / with a 200.
• ruff@0.14.1 format --check and ruff@0.14.1 check pass.

Draft article: https://realpython.com/pbjt/blog/post/2457/change/
Maintenance card: https://trello.com/c/iUHUfici

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