Running `poetry install` with later (>= 1.8) versions of poetry results
in the following dire warning:

    Warning: The current project could not be installed: No file/folder found for package ucam-card-api
    If you do not want to install the current project use --no-root.
    If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
    In a future version of Poetry this warning will become an error!

Heed the warning and add `package-mode = False` into `pyproject.toml`.
This is correct because we use poetry only for dependency management in
our webapps; we do not use it to actually build or distribute any
packages.