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/d20/ex2/ex2.py

@@ -5,7 +5,6 @@ import itertools
 import math
 import sys
 from collections import defaultdict
-from copy import deepcopy
 from typing import Dict, Iterator, List, Set, Tuple
 
 Tile = List[List[str]]