input: Write a python function to filter strings that contain only digits.
  assert filter_digits(["123", "abc", "456", "78a", "90"])==["123", "456", "90"]
  assert filter_digits(["1a2b", "333", "4d5", "6789"])==["333", "6789"]
  assert filter_digits(["abc", "def", "ghi"])==[]
output: def filter_digits(strings):
      return [s for s in strings if s.isdigit()]