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/d10/ex2/ex2.py

@@ -4,7 +4,6 @@ import sys
 from cmath import phase
 from itertools import groupby
 from math import gcd, pi
-from pprint import pprint
 from typing import NamedTuple, Set, Tuple