Update QA tool chain and add Python 3.12

[davidorme]

Description

This PR:

• Simplifies the pytest config to remove mypy and flake8: runs faster, less cruft.
• Replaces flake8, isort, pyupgrade, black in pre-commit with ruff: runs faster, less cruft
• Removes setup.cfg - the new QA chain can all be configured using pyproject.toml.
• Updates VSCode extension recommendations and settings to match
• Fixes a bunch of minor formatting and linting issues that ruff spots but flake8 doesn't.

This PR is initially about checking it all works across platforms and rolling out adoption of the new QA chain. Note that the improved runtime applies to local work and the runner speeds for GA workflows.

This PR does not:

• Update the documentation to match this practice - there is a separate plan to completely overhaul that, which would also tackle these changes.

Fixes #432

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [x] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [x] All tests pass: $ poetry run pytest

Further checks

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works

[davidorme]

@alexdewar and @dalonsoa Can you take an initial look at this and see if it fits with your experience of best practice in migrating away from flake8 etc to ruff.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.61%. Comparing base (9b6cba2) to head (0b49fc0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #433 +/- ## ======================================== Coverage 94.61% 94.61% ======================================== Files 70 70 Lines 3750 3750 ======================================== Hits 3548 3548 Misses 202 202 ```

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