Modernise Packaging and CI

[corranwebster]

This PR:

• [x] removes setup.py and uses pyproject.toml instead (version information is now only via importlib.metadata)
• [x] runs CI using PyPI and pip with Python 3.8 and 3.10 - matrix is Pyside2, PySide6, PyQt5 on all platforms; wxPython on windows only (due to problems installing wheels)
• [x] updates EDM-based ci to use Python 3.8 and PySide6 only
• [x] adds a github action for installing Qt dependencies on ubuntu (copied from TraitsUI)
• [x] switches to using qt as the name of the qt toolkit instance instead of qt4 (but the latter still works, we just prefer ETS_TOOLKIT=qt not ETS_TOOLKIT=qt4) - fixes #1194
• [x] fixes #1207
• [x] cleans up the SplitEditorAreaPane tests and generally makes Tasks destroy methods idempotent, which helps test robustness and fixes #1107

This will need an update to the expected tests before merging.

[mdickinson]

There are a couple of references to setup.py turning up in random places. .gitignore is one.

[mdickinson]

I think we can get rid of .gitattributes at this point, too.

[corranwebster]

Not specifying all checks to be required for now, but doing a representative cross section - we are likely to end up with a minimal set run in PR checks in the future and then a full matrix across all everything we intend to support run as weekly integration tests.