ci: Update tox and pin packaging version for tox

[alexander-alderman-webb]

Description

Fix exception when running populate_tox.py in CI by pinning packaging. See https://github.com/getsentry/sentry-python/actions/runs/21345873752.

Bump actions/checkout in the test group template to match current ./github config files.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

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New Features ✨

• feat(anthropic): Set system instruction attribute by alexander-alderman-webb in #5353
• feat(asyncio): Allow to turn task spans off by sentrivana in #5367
• feat(gen_ai): add function set_conversation_id and managing functions on the Scope and apply it on the Span on .finish() by constantinius in #5362
• feat(google-genai): Set system instruction attribute by alexander-alderman-webb in #5354
• feat(integrations): openai-agents streaming support by constantinius in #5291
• feat(langchain): Set system instruction attribute by alexander-alderman-webb in #5357
• feat(openai-agents): Set system instruction attribute by alexander-alderman-webb in #5355
• feat(pydantic-ai): Set system instruction attribute by alexander-alderman-webb in #5356

Internal Changes 🔧

• ci(release): Fix changelog-preview permissions by BYK in #5368

• ci: Update tox and pin packaging version for tox by alexander-alderman-webb in #5381

• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

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_{🤖 This preview updates automatically when you update the PR.}

[sentrivana]

Thanks for pinning. Looks like new mcp fails the test suite though.

I'll look into removing the pin in the meantime. We're misusing the Version class a bit, so that will need updating.

[sentrivana]

packaging fix: #5382