use pixi, add types, python 3.9-3.14

[bollwyvl]

references

• continues/includes kick py3.13 #159
• starts Update testing strategies for CI performance and robustness #154

changes

• use pixi
  • add pixi.toml
  • check with mypy
  • add test_repo.py to validate deps, versions, etc. between (pyproject|pixi).toml, (.readthedocs|test).yml
• update packaging metadata
  • add trove classifiers
• update ci
  • test
  • release
• src
  • add py.typed
  • update types to match upstreams where possible
  • update CLI parsing with -- to allow using flags "owned" by importnb (-h)
  • utils
    • add initial importnb.utils.coverage plugin
    • require explicitly enabling pytest plugin with -p=importnb.utils.pytest_importnb
• update docs
  • README.md
    • note coverage and pytest integration techniques
    • fix (and test) basic loader info
  • add CHANGELOG.md
  • build and host conda packages along with docs
  • add jupyterlite
    • use a custom pyodide-lock.json
    • add interesting demo examples
      • pytest and coverage
      • sphinx and autodoc
      • ipywidgets
      • ???
    • test examples for restart-and-run-all in browser
  • use mkdocstrings effectively