Project tidying

[dimbleby]

bits and pieces of tidying in pyproject.toml and dependencies

• make ruff configuration consistent with the main project
• make mypy configuration consistent with the main project
• remove unused fixtures, and unused dependency virtualenv (though it shows up via pre-commit anyway)

[dimbleby]

apparently I forgot pytest --integration, so virtualenv and the fixtures had better go back

[dimbleby]

guessing the pipeline failure is because github have changed something about the environment that they are providing for macos, and the CI cache does not spot the difference