Support Django 6.x by widening version constraint

[joseanoxp]

Fixes: #62

Django 6.0 was released in December 2024 (with 6.0.1 in January 2026, and 6.0.5 currently on PyPI). The constraint django>=3.2,<6 prevents pip install identity[django] alongside Django 6+ — exactly the resolver error reported in #62.

After reviewing identity/django.py, this library only uses stable Django APIs that have no breaking changes in Django 6.0:

• django.shortcuts.redirect, render
• django.urls.include, path, reverse
• request.session, request.GET, request.build_absolute_uri(), request.get_full_path()

This PR widens the constraint to django>=3.2,<7 (covering the 6.x line) and adds Python 3.13 to the classifiers (Django 6.0 requires Python 3.12+).

Tested

Validated end-to-end with Django 6.0.5 in a fresh Python 3.12 venv:

1. pip install "identity[django]" resolved without conflict — Django 6.0.5 was selected.
2. from identity.django import Auth and all its methods load.
3. A minimal Django 6 project wiring AUTH = Auth(...) in settings.py and settings.AUTH.urlpattern in urls.py passes python manage.py check cleanly (0 issues).
4. URL resolution: /login, /logout and /getAToken correctly bind to Auth.login, Auth.logout, Auth.auth_response.
5. Both bundled templates (identity/login.html, identity/auth_error.html) load via Django's APP_DIRS loader and render with realistic context.

A parallel PR exists in the Azure-Samples mirror: Azure-Samples/ms-identity-python#3.

References:

• Django 6.0 release notes
• Django Python version support