CI: Fix nightly job, and a few cleanups about zope.testrunner removal

[amotl]

Problem

The nightly CI job started failing yesterday.

• https://github.com/crate/crate-python/actions/workflows/nightly.yml
• https://github.com/crate/crate-python/actions/runs/20085471292

Resolved

• The nightly CI job works again, triggered manually.
  https://github.com/crate/crate-python/actions/runs/20117852553

What's inside

Other than the fix for the problem at hand, the patch also includes a few more other cleanups.

• CI: Fix nightly job, and synchronize task invocations
• CI: Synchronize Python versions between "on-pr" vs. "nightly" jobs
• Tests: Add pytest configuration snippet to pyproject.toml
• Tests: Phase out zope.testrunner completely
• Tests: Use pytest-cov, and configure in pyproject.toml
• Dependencies: Remove uv.lock file
• Dependencies: verlib2 is a runtime dependency

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix-ci-and-cleanups

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