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).

2020/d18/ex2/ex2.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 import sys
-from typing import Callable, Dict, List
+from typing import List
 
 """
 E : T [ * T ]*