game-data-packager git stream stacktrace

[Phil Morrell <debian@emorrp1.name>]

Hello Simon,

After building the latest git master I am unable to create .debs from
GOG archives due to the stacktrace below. I've traced this with git
bisect to commit 2b5a38e but couldn't find the error - as this commit
is newer than v44 in unstable I've not reported this in the BTS,
please let me know if there's any further diagnosis I can help you
with.
--
Phil Morrell

commit 2b5a38e03747045c1bb7844c3a522821213cb841
Author: Simon McVittie <smcv@debian.org>
Date:   Mon Jan 25 10:19:54 2016 +0000

    Distinguish between files and groups

    We no longer need to (ab)use WantedFile to represent a group, and can
    use a smaller, more targeted object which does not support nonsensical
    operations like setting the md5sum of a group.

$ # both give identical stacktraces
$ game-data-packager ultima7 setup_ultima7_serpent_isle_2.1.0.18.exe
$ game-data-packager theme-hospital setup_theme_hospital_2.1.0.8.exe
Traceback (most recent call last):
  File "/usr/lib/python3.4/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.4/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/share/games/game-data-packager/game_data_packager/command_line.py",
line 295, in <module>
    run_command_line()
  File "/usr/share/games/game-data-packager/game_data_packager/command_line.py",
line 288, in run_command_line
    with game.construct_task() as task:
  File "/usr/share/games/game-data-packager/game_data_packager/__init__.py",
line 1224, in construct_task
    self.load_file_data()
  File "/usr/share/games/game-data-packager/game_data_packager/__init__.py",
line 1002, in load_file_data
    self._populate_groups(stream)
  File "/usr/share/games/game-data-packager/game_data_packager/__init__.py",
line 948, in _populate_groups
    stream.seek(0)
io.UnsupportedOperation: underlying stream is not seekable