input: Write a python function to find the most frequent letter in a string, ignoring case and non-alphabet characters.
  assert most_frequent_letter('Hello World!') == ('l', 3)
  assert most_frequent_letter('Testing, testing, 1, 2, 3...') == ('t', 4)
  assert most_frequent_letter('A quick brown fox jumps over the lazy dog.') == ('o', 4)
output: def most_frequent_letter(s):
      from collections import Counter
      s = ''.join(filter(str.isalpha, s)).lower()
      counter = Counter(s)
      most_common = counter.most_common(1)[0]
      return most_common