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
 

2020/d18/ex2/ex2.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 import sys
-from typing import Callable, Dict, List
+from typing import List
 
 """
 E : T [ * T ]*

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]]

2020/d23/ex1/ex1.py

@@ -2,7 +2,7 @@
 
 import itertools
 import sys
-from typing import List, Tuple
+from typing import List
 
 
 def solve(circle: List[int]) -> int:

2020/d23/ex2/ex2.py

@@ -2,7 +2,7 @@
 
 import itertools
 import sys
-from typing import List, Tuple
+from typing import List
 
 
 def solve(circle: List[int]) -> int:

2020/d24/ex2/ex2.py

@@ -3,7 +3,6 @@
 import itertools
 import sys
 from collections import defaultdict
-from copy import deepcopy
 from typing import Dict, Iterator, List, Tuple
 
 Offset = Tuple[int, int]