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/d11/ex1/ex1.py

@@ -2,7 +2,6 @@
 
 import itertools
 import sys
-from copy import deepcopy
 from typing import Iterator, List, Set, Tuple
 
 Grid = List[List[int]]

2021/d11/ex2/ex2.py

@@ -2,7 +2,6 @@
 
 import itertools
 import sys
-from copy import deepcopy
 from typing import Iterator, List, Set, Tuple
 
 Grid = List[List[int]]