input: Write a python function to find the most frequent letter in a given string.
  assert most_frequent_letter("foo bar baz foo") == 'o'
  assert most_frequent_letter("aabbccddeeffgg") == 'a'
  assert most_frequent_letter("xyzxyzxyz") == 'x'
output: def most_frequent_letter(s):
      from collections import Counter
      s = s.replace(" ", "")
      frequency = Counter(s)
      most_frequent = max(frequency, key=frequency.get)
      return most_frequent