treewide: fix 'ruff check' errors

This is mostly about unused imports.

A couple errors remain, but are fine in my book (using `l` as a variable
name, assigning a lambda to a variable).

2022/d21/ex2/ex2.py

@@ -93,7 +93,7 @@ class MathObserver:
 
 class Monkey:
     def get_value(self, monkeys: dict[str, "Monkey"]) -> Num:
-        raise NotImplemented
+        raise NotImplementedError
 
 
 @dataclasses.dataclass