How to write pbm file in Python 3?

[André]

I'm trying to create pbm (portable bitmap) files using Python 3 and
have run into a problem.   The example I am using is the Python 2
Mandelbrot program found at
http://shootout.alioth.debian.org/u32/program.php?test=mandelbrot&lang=python&id=1

When I run it using Python 2 with size=100, I get a file of size 1311
bytes.  This file contains the correct image.

When I run it using Python 3 with the same parameter (and replacing
xrange by range - the only change suggested by 2to3), I get a file of
size 1812 bytes; this file does not contain the right image.

Any help would be much appreciated.

André