ci: trigger on merge_group so a merge queue can be enabled

[wbarnha]

Description

Makes master merge-queue-ready. A GitHub merge queue validates each candidate as a merge_group event, but the workflow only triggered on push / pull_request / release — so the required check job would never run for a queued PR and the queue would stall indefinitely. This adds the merge_group trigger:

on:
  push:
    branches: [master]
  pull_request:
    branches: [master]
  merge_group:          # <-- added
  release:
    types: [created]

• No behaviour change for normal pushes/PRs.
• The check job (needs: [lint, test-pytest], alls-green) is already the single aggregated required status a queue wants, and now runs on merge_group too.
• The advisory jobs (test-integration, test-pypy, test-redis-integration) stay continue-on-error / out of the check gate, so they won't block the queue.

To actually turn the queue on (maintainer, repo settings — not doable from a PR)

Settings → Rules / Rulesets (or branch protection on master) → enable "Require merge queue" targeting master, with the required status check set to ✅ Ensure the required checks passing (the check job). Pick merge method / max batch size as desired. This PR is the prerequisite; the toggle itself is admin-only.

Top of the stacked chain (based on #725). YAML validated.

🤖 Generated with Claude Code

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