Fix deprecated setup.py commands, broken publish job, and README drift

[scottclowe]

Summary

Fixes several things in the template that are broken, deprecated, or out of date.

Packaging

• Remove the test and upload custom commands from setup.py. The setuptools test command has been removed, and building with current setuptools (84.0.0) warns that references to it will cause build errors in the future.
• README: recommend pip install --editable . and python -m build instead of invoking setup.py directly.

CI

• test.yaml: remove the branches key under schedule. It is not valid there, and scheduled runs always use the default branch.
• test-release-candidate.yaml:
  • Remove the duplicated macos-latest / 3.12 matrix entry.
  • Build wheels with python -m build --wheel. Wheels are still built on every OS and Python version, since a package made from the template may contain compiled extensions. The sdist is now built once, in the publish job.
  • Fix the publish job's artifact download. download-artifact@v4 needs pattern: wheel-* with merge-multiple: true; name: wheel-* matches nothing.
  • Publish to Test PyPI with Trusted Publishing (id-token: write, testpypi environment) instead of an API token, and replace the deprecated repository_url input with repository-url.

Docs

• Remove the stale line-number links from the README.
• Replace the black_nbconvert reference with nbQA, which is what the pre-commit config actually uses.
• Remove the section on the nonexistent lint workflow (the README said five workflows; there are four).
• Correct the claim that PyPI releases cannot easily be yanked. The real hazard is that a version number can never be reused.
• Document the wheel caveats: wheels are identical for pure-Python packages, and compiled packages need manylinux wheels for PyPI.
• Update the "Other CI/CD options" list: drop Travis CI and AppVeyor, add GitLab CI/CD.

Testing

• python -m build succeeds with setuptools 84.0.0, with no test-command deprecation warning.
• pre-commit passes on every commit.
• Workflow YAML parses and setup.py compiles at every commit.
• The workflows have not been run end to end. The publish job is still disabled by default, and enabling it requires registering a trusted publisher on Test PyPI first.

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