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("world")==1
  assert count_vowels("aeiou")==5
  assert count_vowels("")==0
  assert count_vowels("Python Programming")==4
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count