Enable ruff-formatter alongside ruff

[jaraco]

In jaraco/skeleton#101, I'm exploring replacing black with ruff formatter. For part of that change, I'd like for the formatter in pytest-ruff to be enabled by default by way of this enabler.

I acknowledge that this change will be substantially backward-incompatable (enforcing formatting where it wasn't previously). However, it is the spirit of this plugin to enable best practices by default and since skeleton is switching, it makes sense for it to be enabled by default.

[jaraco]

To restore the prior behavior, add this to a project's pyproject.toml:

[tool.pytest-enabler.ruff]
addopts = "--ruff"