input: Write a python function to count uppercase and lowercase letters in a string.
  assert countLetters('Hello World') == (2, 8)
  assert countLetters('Python is Awesome') == (2, 13)
  assert countLetters('UPPER lower') == (5, 5)
output: def countLetters(s):
      upper_count = sum(1 for char in s if char.isupper())
      lower_count = sum(1 for char in s if char.islower())
      return (upper_count, lower_count)