input: Write a python function to find the most common letter in a given string.
  assert most_common_letter("this is a sample sentence") == 'e'
  assert most_common_letter("hello world") == 'l'
  assert most_common_letter("abracadabra") == 'a'
output: def most_common_letter(s):
      from collections import Counter
      count = Counter(s.replace(" ", ""))
      return count.most_common(1)[0][0]