Create a function in Python that given a string, calculates the number of vowels in it.
Hello World!
def count_vowels(s):
  vowels = 'aeiou'
  count = 0
  for c in s.lower():
    if c in vowels:
      count += 1
  return count

s = 'Hello World!'
print(count_vowels(s)) # Output 3