Change to python-2.5 or later relative paths

[dmitry-g]

Herd clients failed loading modules:

  File "herd/murder_client.py", line 22, in <module>
    from BitTornado.download_bt1 import (
  File "herd/BitTornado/download_bt1.py", line 7, in <module>
    from BT1.Choker import Choker
  File "herd/BitTornado/BT1/Choker.py", line 5, in <module>
    from herd.BitTornado.clock import clock
  File "herd/herd.py", line 13, in <module>
    import BitTornado.BT1.track as bttrack
  File "herd/BitTornado/BT1/track.py", line 11, in <module>
    from T2T import T2TList
  File "herd/BitTornado/BT1/T2T.py", line 4, in <module>
    from Rerequester import Rerequester
  File "herd/BitTornado/BT1/Rerequester.py", line 5, in <module>
    from herd.BitTornado.zurllib import urlopen, quote

Fortunately, there is a fix for that at russss#29 though it hasn't been merged to the original project.

This PR applies the fix to our fork.

@vinted/sre