input: Write a function to count the number of vowels and consonants in a given sentence.
  assert count_letters("hello world") == (3, 7)
  assert count_letters("Python programming is fun!") == (6, 14)
  assert count_letters("A quick brown fox jumps over the lazy dog") == (11, 24)
output: def count_letters(sentence):
      vowels = "aeiouAEIOU"
      vowel_count = 0
      consonant_count = 0
      for char in sentence:
          if char.isalpha():
              if char in vowels:
                  vowel_count += 1
              else:
                  consonant_count += 1
      return vowel_count, consonant_count