input: Write a python function to count the number of vowels in a given string.
  assert count_vowels("hello")==2
  assert count_vowels("programming")==3
  assert count_vowels("AEIOUaeiou")==10
output: def count_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count