Repair CI and support current Django (5.1-6.1)

[PetrDlouhy]

Independent of #204, but found through it. Two layers, one commit each.

Commit 1 — repair what's red on every PR today

tox (3.12) — the workflow has run Python 3.12 since #202, but [gh-actions] in
tox.ini was never taught the mapping, so 3.12 falls back to the bare unpinned py env.
It installed whatever Django was newest; since Django 6 released, makemigrations --check
fails before a single test runs. tox (3.7) — ubuntu-latest runners no longer ship
Python 3.7 (Version 3.7 with arch x64 not found); dropped from the workflow, the local
tox envs stay.

Commit 2 — support current Django, extend the matrix

Everything the matrix tested (2.2–5.0) is past end-of-life; the currently supported
Djangos are 5.2 LTS, 6.0 and 6.1. Letting the resolver install them surfaced two real
package bugs, not just config:

• Django 6.1 gave SessionBase a __bool__ — an empty session is now falsy. Two
  truthiness checks changed meaning: _get_participant demoted every fresh visitor to a
  DummyUser (nothing enrolled, nothing counted), and _session_key returned None for
  them, keying every such visitor's enrollments and counters to the same identifier —
  the suite's MultipleObjectsReturned was that collision. Both are identity checks now.
  Without this, the package silently stops counting new visitors on Django 6.1.
• Django ≥ 6 defaults DEFAULT_AUTO_FIELD to BigAutoField, making makemigrations
  demand an Alter field id on enrollment migration from every project. The AppConfig now
  pins the historical AutoField, so existing installations aren't asked to rewrite their
  tables. (Whether the project wants to move to BigAutoField eventually is a real
  question — but that's a schema decision for a maintainer, not a side effect of CI.)

Matrix: django5.1/5.2 on py310–313, django6.0/6.1 on py312–313, Python 3.13 added to
the workflow and [gh-actions].

Verification

Full suite run locally on Django 5.0.14, 5.1.15, 5.2.17, 6.0.8 and 6.1 —
makemigrations --check clean and all tests OK on each.

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📥 Commits

Reviewing files that changed from the base of the PR and between df6fe5d and 6e93e9e.

📒 Files selected for processing (4)

• .github/workflows/tests.yml
• experiments/apps.py
• experiments/utils.py
• tox.ini

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