* #154: Support for gzip levels has been made more consistent. Levels 0-9
  are supported. Level 11 which was only available when the ``pigz`` backend was
  present is not supported anymore. Level 0, gzip format without compression,
  lead to crashes when the ``gzip`` application backend was used as this does
  not have a ``-0`` flag. ``xopen()`` now defers to other backends in that case.
* #152: ``xopen()`` now accepts `file-like objects
  <https://docs.python.org/3/glossary.html#term-file-object>`_ for its filename
  argument.
* #146, #147, #148: Various refactors for better code size and readability:

    * PipedCompressionReader/Writer are now combined _PipedCompressionProgram
      class.
    * _PipedCompressionProgram is binary-only. For text reading and writing
      it is wrapped in an ``io.TextIOWrapper`` in the ``xopen()`` function.
    * Classes that derive from PipedCompressionReader/Writer have been removed.
* #148: xopen's classes, variables and functions pertaining to piped reading
  and writing are all made private by prefixing them with an underscore.
  These are not part of the API and may change between releases.