Is pangram?

Write a function `is_pangram(s: str) -> bool` that returns `True` if the given string `s` is a pangram, meaning it contains every letter of the English alphabet (`a` through `z`) at least once. The check must ignore letter case (both uppercase and lowercase count) and all characters that are not alphabetic (such as spaces, digits, punctuation). The function should return `False` for empty strings or strings that miss any alphabet letter. **Function signature:** `def is_pangram(s: str) -> bool:` **Examples:** - `is_pangram("The quick brown fox jumps over the lazy dog")` returns `True`. - `is_pangram("Hello world")` returns `False` because many letters are missing.

>>> is_pangram("The quick brown fox jumps over the lazy dog")
True
>>> is_pangram("Hello world")
False
>>> is_pangram("Pack my box with five dozen liquor jugs")
True