Python - Lectura y union de dos txt

def intercalar_txt(file1, file2, file3 = 'nuevo.txt'):

    with open(file1, 'r') as f:

        lines1 = f.readlines()[::2]

    with open(file2, 'r') as f:

        lines2 = f.readlines()[1::2]

    lines3 = []

    nlimit = 0

    for nline in range(len(min(lines1, lines2, key = len))):

        lines3.extend(lines1[nline] + lines2[nline])

        nlimit += 1

    lines3.extend(max(lines1, lines2, key = len)[nlimit:])

    with open(file3, 'wb+') as f:

        f.write(''.join(lines3).rstrip('\n'))

intercalar_txt('txt1.txt', 'txt2.txt', 'txt3.txt')

with open(file3, 'wb+') as f:

with open(file3, 'w+') as f:

f.write(''.join(lines3).rstrip('\n'))

f.write((''.join(lines3).rstrip('\n')).encode())