Upgrade build system from PyScaffold 3.3 to 4.6 (PEP 517)

[bakcsa83]

Summary

• PyScaffold 3.x relied on setup_requires and easy_install, which are broken in modern Python environments and forced users to set PIP_USE_PEP517=1 to install. This upgrade moves to a PEP 517/518 build system so pip install works out of the box.
• Replace pyscaffold build dependency with setuptools_scm declared directly in pyproject.toml
• Switch from deprecated pkg_resources to importlib.metadata (stdlib since 3.8)
• Use python -m build in CI instead of python setup.py sdist/bdist_wheel
• Bump GitHub Actions (checkout v4, upload-artifact v4, setup-miniconda v3)
• Drop Python 3.6 from CI matrix (EOL); minimum Python version is now 3.8

[bakcsa83]

Hi Christoph,
The motivation for this PR is that datedown is a transitive dependency of QA4SM, and its outdated build system is causing unnecessary complications.

If you could merge these changes and release a new pypi version that would be great.

Cheers,
Zoltan