input: Write a function to count the number of vowels in a given string. If the string is empty, return 0.
  assert count_vowels("hello")==2
  assert count_vowels("python programming")==4
  assert count_vowels("AEIOUaeiou")==10
  assert count_vowels("")==0
  Assert count_vowels("Th1s 1s a t3st!")==1
output: def count_vowels(s):
      vowels = "AEIOUaeiou"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count