import fileinput
import os

with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
    for i, line in enumerate(file, start=1):
        if i & 1: # odd
            print(line, end='') # keep line (stdout is redirected to the file)
os.unlink(filename + '.bak') # remove the backup on success

import sys
from itertools import islice

sys.stdout.writelines(islice(file, 0, None, 2))  # keep lines[::2]

with open(filename) as file:
    lines = file.readlines()[::2] # lines to keep
with open(filename, 'w') as file:
    file.writelines(lines)

from itertools import islice

with open(filename, 'r+') as file:
    write_offset = file.tell() # where to write next
    for line in islice(iter(file.readline, ''), 0, None, 2):  # keep lines[::2]
        read_offset = file.tell() # where to read next
        file.seek(write_offset)
        file.write(line)
        write_offset = file.tell()
        file.seek(read_offset)
    file.truncate(write_offset)