Use dependency groups (PEP 735)

[mwtoews]

This PR moves some dependency lists from project.optional-dependencies (aka "extras") to dependency-groups, as described by PEP 735. Some details:

• Keep pyqt5 and pyside6 as "extras" intended for end-users
• Move test, docs and cov to dependency groups, intended for developers
  • Note that pip install --dryrun ipykernel[test] will show a warning that the package does not have an extra named test.
  • Developers that need this dependency will require pip v25.1 (2025-04-26) (or uv) and use the --group test option
• Add a dev group, which adds pre-commit and the test group; uv enables this group by default.
• Hatch has supported dependency groups since v1.16.0, which is within the version specified in the build requirements
• Dependency lists are indented constantly and sorted