build: tox pin django 5.2-5.3

[marlonkeating]

What

The pii_check CI job fails with:

ImportError: cannot import name 'cc_delim_re' from 'django.utils.cache'

Why

requirements/test.txt deliberately omits the Django== pin — the Makefile
strips it out (# Let tox control the Django version for tests) so that each
[testenv] factor (e.g. django52) can supply its own Django>=5.2,<5.3
constraint before installing test.txt.

[testenv:pii_check] is a standalone tox section that never inherited that
constraint, so Django was left completely unpinned in that one job. Once
Django 6.1 landed on PyPI, pip resolved to it for pii_check, but 6.1 removed
cc_delim_re from django.utils.cache, which the pinned
djangorestframework==3.17.1 still imports — breaking
enterprise/admin/views.py's import chain during django_find_annotations.

This isn't caused by any recent requirements upgrade — requirements/*.txt
are unchanged; it's a latent gap in tox.ini that surfaced only once a new
Django release appeared. The edx-requirements-bot PR #2675 hits the same
failure for the same reason.

Fix

Add the same Django>=5.2,<5.3 pin already used by the django52 env to
[testenv:pii_check]'s deps.

Testing

Ran tox -e pii_check locally against this branch:

• Before: fails with the cc_delim_re ImportError
• After: Django resolves to 5.2.16; django_find_annotations linting and
  PII coverage checks (85.1%) pass

[marlonkeating]

Fixed by #2551

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.02%. Comparing base (99c7e2c) to head (423fe8b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2677      +/-   ##
==========================================
+ Coverage   86.97%   87.02%   +0.04%     
==========================================
  Files         261      262       +1     
  Lines       17143    17228      +85     
  Branches     1693     1705      +12     
==========================================
+ Hits        14910    14992      +82     
- Misses       1896     1897       +1     
- Partials      337      339       +2     

Flag	Coverage Δ
unittests	87.02% <ø> (+0.04%)	⬆️

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