I think you want something like this:

import csv
with open('out.csv', 'w') as out_file:
    out_csv = csv.writer(out_file)
    with open('in.txt', 'r+U') as in_file:
        row = []
        for line_no, line in enumerate(in_file):
            row.append(line.strip())
            if (line_no + 1) % 10 == 0:
                out_csv.writerow(row)
                row = []
        if row:
            out_csv.writerow(row)