input: Write a python function to count the number of vowels in a sentence.
  assert count_vowels("This sentence has five vowels") == 7
  assert count_vowels("Hello World") == 3
  assert count_vowels("Python Programming") == 4
output: def count_vowels(sentence):
      vowels = "aeiouAEIOU"
      count = sum(1 for char in sentence if char in vowels)
      return count