Add ruff linting

Configure ruff in pyproject.toml and apply its fixes: modern annotations
(X | None, builtin generics, collections.abc), sorted imports, explicit
stacklevel in test warnings.

pyproject.toml

@@ -57,6 +57,12 @@ warn_unused_ignores = true
 module = ["paramiko", "systemd", "dbus"]
 ignore_missing_imports = true
 
+[tool.ruff]
+line-length = 120
+
+[tool.ruff.lint]
+select = ["E", "F", "W", "I", "B", "UP"]
+
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 # Import the repo copy, not an installed one