input: Write a function to count the total number of vowels in a given string. Vowels are defined as 'a', 'e', 'i', 'o', 'u' regardless of their case. If the given string is empty then return 0.
  assert count_vowels("Hello World")==3
  assert count_vowels("Python")==1
  assert count_vowels("AEIOU")==5
  assert count_vowels("bbccdd")==0
  assert count_vowels("")==0
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count