restore support for bz2 compression

[nhoffman]

In the migration to python3, support for bz2 compression was dropped, for two reasons:

• bz2.BZ2File() opens files in binary mode only
• bz2.open() returns an object with no name attribute

These two issues made it difficult to read and write bz2-compressed files using the FileType object. I decided to drop bz2 compression in the interest of making it to python3. Perhaps someone would like to bring it back...