input: Write a python function to find the most common letter in a string.
  assert most_common_letter("hello world") == 'l'
  assert most_common_letter("test case") == 't'
  assert most_common_letter("aabbcc") == 'a'
output: def most_common_letter(s):
      from collections import Counter
      counter = Counter(s.replace(" ", ""))
      return counter.most_common(1)[0][0]