input: Write a function to count the total number of vowels in a given string and return the count. A vowel is defined as one of the characters in the set "aeiou".
  assert count_vowels("Hello World")==3
  assert count_vowels("Python Programming")==4
  assert count_vowels("This is a test")==4
  assert count_vowels("AEIOU")==5
  assert count_vowels("")==0
output: def count_vowels(s):
      vowels = "aeiou"
      count = 0
      for char in s.lower():
          if char in vowels:
              count += 1
      return count