import-images fails under Python 3 when there are multiple images and at least one lacks a base

[Colin Watson]

https://lists.launchpad.net/ubuntu-phone/msg10107.html reports an import-images failure following the conversion to Python 3 in lp:~ubuntu-system-image/ubuntu-system-image/server r246:

Traceback (most recent call last):
  File "/srv/system-image.ubuntu.com/bin/import-images", line 297, in <module>
    pub.sync_aliases(channel_name)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/tree.py", line 762, in sync_aliases
    self.sync_alias(alias_name)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/tree.py", line 668, in sync_alias
    for image in sorted(target_images - device_images):
TypeError: unorderable types: int() < NoneType()

This is due to this change in Python 3:

  https://docs.python.org/3/whatsnew/3.0.html#ordering-comparisons

I've temporarily cowboyed a server change to revert import-images to Python 2 since presumably the images containing non-ASCII files have been processed by now (I hope), but this should be fixed properly. I don't know whether the best answer is to temporarily map None onto some int value, or to create a new class with custom comparator methods, or something else.