nix: migrate to 'ruff'

Having just the one tool is better.

Incidentally, this fixes the ternary formatting, which was messed up in
recent `black` versions.

ruff.toml

@@ -0,0 +1,8 @@
+[lint]
+extend-select = [
+    "I", # Import sorting is a lint check
+]
+ignore = [
+    "E731", # Assigning lambdas is sometimes more readable
+    "E741", # Ambiguous variable names are good enough actually
+]