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/d16/ex2/ex2.py

@@ -6,7 +6,7 @@ import itertools
 import math
 import sys
 from dataclasses import dataclass
-from typing import Callable, Dict, Iterable, Iterator, List, Tuple, TypeVar
+from typing import Callable, Dict, Iterable, Iterator, List, Tuple
 
 RawPacket = List[bool]