Redesign PyPy 3.9 CI pipeline: drop dep pins, add step-level failure handling

[Copilot]

PyPy 3.9 fails CI both at install time (PyO3 incompatibilities with cryptography≥45 and nh3≥0.3) and at test time (4 persistent failures). The previous fix pinned those deps in requirements/test.txt, which is the wrong layer — the pins don't even eliminate test failures.

Changes

• requirements/test.txt: Remove cryptography<45 and nh3<0.3 PyPy markers. Dep constraints for a flaky optional CI target don't belong in the shared requirements file.

• .github/workflows/python-package.yml: Restructure test-pypy to degrade gracefully at the step level:

  • Install dependencies gets id: install + continue-on-error: true — install failures no longer cascade
  • Run tests and Enforce coverage gain if: steps.install.outcome == 'success' — skipped cleanly if install fails, no spurious errors

The job already carries continue-on-error: true at the job level and is excluded from the required check (needs: [lint, test-pytest]), so any PyPy failure — install or test — is fully non-blocking.