If given, compression 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.