Add support for newer versions of Jupyter ecosystem

[yanovs]

• Add support for nbconvert >= 6.0.0, including support for the new default template lab, the new light/dark themes (for lab-based templates), and the legacy template classic (both for nbconvert>=5.6.1,<6.0.0 and explicit template_name = 'classic' in nbconvert>=6.0.0)
• Add new template option: lab_narrow, which is based on the lab template but retains the centered, narrow body of the classic template (also supports light/dark themes)
• Add systematic testing of different (and newer) bundles of packages
• Fix when images are base64-encoded (instead of raw bytes)
• Fix inline mode when using static_ipywidgets
• Enable all linters
• Add py.typed
• Clean up demo notebooks and set up static demo page
• Simplify CI by switching from tox-gh-actions to tox labels (available in tox >= 4.0.0)