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/d07/ex1/ex1.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 
-import itertools
 import sys
 from typing import List
 

2021/d07/ex2/ex2.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 
-import itertools
 import sys
 from math import ceil, floor
 from typing import List