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/d17/ex1/ex1.py

@@ -1,7 +1,6 @@
 #!/usr/bin/env python
 
 import itertools
-import math
 import sys
 from typing import Iterator, List, NamedTuple
 

2021/d17/ex2/ex2.py

@@ -1,7 +1,5 @@
 #!/usr/bin/env python
 
-import itertools
-import math
 import sys
 from typing import Iterator, List, NamedTuple