Strings: valid palindrome

A palindrome reads the same forwards and backwards: "level", "noon".

Strings in Python support handy tools:

s = "Hello"
s.lower()        # "hello"
s[::-1]          # "olleH"   — slicing with step -1 reverses
"a1".isalnum()   # checks letters/digits, char by char

This is LeetCode #125. We only care about letters and digits, and we ignore case. So "A man, a plan, a canal: Panama" is a valid palindrome.

Your task

Write is_palindrome(s) that returns True if s is a palindrome once you:

1. drop every character that is not a letter or digit, and
2. compare case-insensitively.

is_palindrome("A man, a plan, a canal: Panama")  # True
is_palindrome("race a car")                        # False
is_palindrome("")                                  # True  (nothing to contradict it)

Hint: build a cleaned list of c.lower() for each c where c.isalnum(), then compare it to its reverse.