Standalone ruff checks + Automatically detect requires-python

[Avasam]

It recently crossed my mind that you don't need to run Ruff on every os and Python version. In fact, you don't even need to have Python installed to run Ruff !

Running it through pytest everytime seems not only wasteful, but has caused issues (between version desync, huge logs spam, and not as clear errors)

astral-sh/ruff-action@v3 automatically finds the version of Ruff to install and run from looking at a pyproject.toml's project.optional-dependencies and dependency-groups.

Since no other test uses Ruff, no need to install it, so I split it into its own extra.

This PR also bumps Ruff's lower bound to 0.11, where the detection of requires-python is automated.

[jaraco]

Probably we want to revert the changes in this file. Ruff won't run on cygwin, and we definitely want pytest-ruff so that ruff runs under pytest. If ruff >= 0.11 is needed, we can just wait for that to become a de facto minimum.

[Avasam]

Part of the proposition here is to not run under pytest, nor use pytest-ruff.
And no cygwin should install this extra.

The version bump is to remove the explicit pyproject.toml reference in ruff.toml. That could be split off.

[jaraco]

Running it through pytest everytime seems not only wasteful, but has caused issues (between version desync, huge logs spam, and not as clear errors)

I don't want to run ruff separately. I really like being able to just run tox to validate that the code meets spec (including ruff). It's easy enough to avoid running it (tox -- -p no:ruff). But it's so fast, it's really fine to have it integrated.

The current design has the pytest plugins (coverage, mypy, ruff) all integrated and using a consistent interface. See #48 for more details.