feat: add Python 3.12 support and drop Python 3.8–3.11

[naincy128]

Summary

Adds support for Python 3.12 and drops support for Python 3.8–3.11.
The CI, tox configuration, and package classifiers have been updated to target Python 3.12 only. Known consumers, edx-analytics-dashboard and course-discovery, already use Python 3.12.

Changes

• tox.ini, ci.yml: test py312 only (was py38 and py312); bump checkout@v4, setup-python@v5 and codecov-action@v5; CI runs on ubuntu-24.04 because ubuntu-20.04 is retired.
• upgrade-python-requirements.yml: the weekly requirements bot compiles with Python 3.12.
• setup.py: Python 3.12 classifier only, and python_requires='>=3.12'.
• requirements/*.txt: regenerated with Python 3.12, which brings in pylint 4 and pytest 9.
• course_summaries.py: silence pylint 4's new R0917 check. The method is public API, so it isn't refactored.

Tests

tox -r
  py312: OK   (72 passed)
  quality: OK (Python 3.12, pylint 4.0.9: 10.00/10, 0 messages across analyticsclient)

Notes

• Codecov: fail_ci_if_error is intentionally false, and the step now passes token: ${{ secrets.CODECOV_TOKEN }}.
  • This repo has no CODECOV_TOKEN secret yet, so uploads without a token are rate-limited (HTTP 429) and were failing CI even though tests passed.
  • Once an admin adds the secret, set fail_ci_if_error back to true.
• Branch protection on master has no required status checks, so the renamed CI jobs don't block the merge.