input: Write a function to count the frequency of each letter in a string using the Counter class from the collections module.
  assert count_letters("This is a sample string") == Counter("This is a sample string")
  assert count_letters("Hello World") == Counter("Hello World")
  assert count_letters("aabbcc") == Counter("aabbcc")
output: from collections import Counter
  
  def count_letters(s: str) -> Counter:
      return Counter(s)