input: Write a function to extract digits from a string and return them as a list of integers.
  assert extract_numbers("a1b2c3") == [1, 2, 3]
  assert extract_numbers("abc") == []
  assert extract_numbers("123abc456") == [1, 2, 3, 4, 5, 6]
output: def extract_numbers(s):
      return [int(char) for char in s if char.isdigit()]