input: Write a function to count the total number of vowels in a given string. The function should return the count of vowels, which are defined as 'a', 'e', 'i', 'o', and 'u'.
  assert count_vowels("hello")==2
  assert count_vowels("world")==1
  assert count_vowels("aeiou")==5
  assert count_vowels("Python")==1
  assert count_vowels("")==0
output: def count_vowels(s):
      vowels = "aeiou"
      count = 0
      for char in s:
          if char.lower() in vowels:
              count += 1
      return count