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/d09/ex1/ex1.py

@@ -1,11 +1,10 @@
 #!/usr/bin/env python
 
-import itertools
 import sys
 from copy import deepcopy
 from dataclasses import dataclass, field
 from enum import IntEnum
-from typing import Callable, List, NamedTuple
+from typing import List, NamedTuple
 
 
 class ParameterMode(IntEnum):

2019/d09/ex2/ex2.py

@@ -1,11 +1,10 @@
 #!/usr/bin/env python
 
-import itertools
 import sys
 from copy import deepcopy
 from dataclasses import dataclass, field
 from enum import IntEnum
-from typing import Callable, List, NamedTuple
+from typing import List, NamedTuple
 
 
 class ParameterMode(IntEnum):