code: def find_first_stop_codon(nucleotide_sequence):
      
      n_mutant_codons = len(nucleotide_sequence) // 3
      for i in range(n_mutant_codons):
          codon = nucleotide_sequence[3 * i:3 * i + 3]
          if codon in STOP_CODONS:
              return i
      return -1
docs: Given a sequence of codons (expected to have length multiple of three),
      return index of first stop codon, or -1 if none is in the sequence.