from Bio import SeqIO

o = open("locus-tags.csv",'w')
o.write("old-tag,new-tag\n")

for seq in SeqIO.parse(open("file.gbk",'r'),'genbank'):
  for f in seq.features:
    if f.type == "gene":
      o.write("{},{}\n".format(f.qualifiers["old_locus_tag"][0],f.qualifiers["locus_tag"][0])

o.close()