* #142: The python-isal compression backend is now only used for compression
  levels 1 and 2. Contrary to other backends, python-isal level 0 gave
  compressed rather than uncompressed data in gzip format. Level 3 on
  python-isal did not provide better compression than level 2.
* #140: PipedCompressionReader/Writer now derive from the `io.IOBase
  <https://docs.python.org/3/library/io.html#io.IOBase>`_ abstract class.
* #138: The gzip default compression level is now 1 when no value is provided
  by the calling function. The default used to be determined by the backend.
* #135: xopen now uses zlib-ng when available and applicable.
* #133: Piped ``igzip`` is no longer used as a (de)compression backend as
  python-isal's threaded mode is a better choice in all use cases.