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).

2021/d06/ex1/ex1.py

@@ -3,7 +3,6 @@
 import itertools
 import sys
 from collections import Counter
-from dataclasses import dataclass
 from typing import Iterator, List, TypeVar
 
 T = TypeVar("T")