Assuming you have enough memory to hold everything in memory:
with open('file.txt', 'r') as fin:
data = fin.read().splitlines(True)
with open('file.txt', 'w') as fout:
fout.writelines(data[1:])
We could get fancier, opening the file, reading and then seeking back to the beginning eliminating the second open
, but really, this is probably good enough.