input: Write a python function to find the longest sequence in a list of sequences.
  assert longest_sequence(["ATG", "ATATCAT", "CGATTGTA"]) == "CGATTGTA"
  assert longest_sequence(["A", "AA", "AAA", "AAAA"]) == "AAAA"
  assert longest_sequence(["GATTACA", "TGCAT", "ATG"]) == "GATTACA"
output: def longest_sequence(sequences):
      if not sequences:
          return None
      return max(sequences, key=len)