TypeError: 'str' does not support the buffer interface

[Michael Terry]

I ran "auto-upgrade-tester ubuntu" in quantal and got the following exception:

Building new image '/var/cache/auto-upgrade-tester/test-image.ubuntu' based on '/var/cache/auto-upgrade-tester//precise-i386.qcow2'
Starting ['kvm', '-hda', '/var/cache/auto-upgrade-tester/test-image.ubuntu', '-monitor', 'stdio', '-localtime', '-no-reboot', '-net', 'nic,model=virtio', '-net', 'user', '-redir', 'tcp:54322::22', '-vnc', 'localhost:1', '-m', '1536']
QEMU 1.1.0 monitor - type 'help' for more information
(qemu) Traceback (most recent call last):
  File "/usr/bin/auto-upgrade-tester", line 170, in testUpgrade
    if not backend.bootstrap():
  File "/usr/lib/python3/dist-packages/AutoUpgradeTester/UpgradeTestBackendQemu.py", line 268, in bootstrap
    etc_hosts.write('127.0.0.1 localhost\n')
TypeError: 'str' does not support the buffer interface
Caught exception in testUpgrade for '/usr/share/auto-upgrade-tester/profiles/ubuntu' ('str' does not support the buffer interface)

I'm working on a patch. I think it just needs to convert str to bytes.