I'll have a patch for this in the next couple of days (and a similar one
for the lzma module, which has the same issue (even though it's not a
regression in that case)).

In the meanwhile, you can work around this by feeding the compressed data
to a BZ2Decompressor yourself - it stops at the end of the bz2 stream,
with any leftover data stored in its 'unused_data' attribute.