input: Write a python function to find the most common letter in a string.
  assert find_most_common_letter("This is a test string") == 's'
  assert find_most_common_letter("hello world") == 'l'
  assert find_most_common_letter("aabbcc") == 'a'
output: def find_most_common_letter(s):
      from collections import Counter
      s = s.replace(" ", "")
      counter = Counter(s)
      most_common = counter.most_common(1)
      return most_common[0][0]