input: Write a python function to find the number of letters that are common to two given strings.
  assert num_common_letters('Hello', 'World') == 2
  assert num_common_letters('Python', 'Java') == 0
  assert num_common_letters('abc', 'cab') == 3
output: def num_common_letters(str1, str2):
      set1 = set(str1.lower())
      set2 = set(str2.lower())
      return len(set1 & set2)