If given, block_size should be a number from 1 to 9 indicating the
block size used when doing compression. The actual block size will
be a 100000 times this number. Low numbers are considered 'fast', higher numbers
are considered 'slow' but give better packing. The parameter is set to
9 if it is omitted.
This function can also be used to re-initialize a Bz2.Deflate object
so it can be re-used.