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).

2019/d13/ex2/ex2.py

@@ -3,7 +3,7 @@
 import sys
 from dataclasses import dataclass, field
 from enum import IntEnum
-from typing import Dict, Iterable, List, NamedTuple, Tuple, TypeVar
+from typing import Iterable, List, NamedTuple, Tuple, TypeVar
 
 
 class ParameterMode(IntEnum):