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).

2020/d07/ex2/ex2.py

@@ -2,8 +2,6 @@
 
 import re
 import sys
-from collections import defaultdict
-from copy import deepcopy
 from dataclasses import dataclass
 from typing import Dict, List, Set, Tuple