Bump python-debian requirement to 0.1.21-nmu2 for Python 3.x compatibility
Change logger.warn() -> logger.warning() (apparently the former is deprecated)
Bug fix for Python 3.x compatibility (missing `str_compatible' decorator)
Bug fix for Python 3.x compatibility (`str_compatible' decorator)
Bug fix for inspect_package() (hard links weren't recognized)
Bug fixes for Python 3.x compatibility (with assorted small changes*)

* Changes like:

 - Bug fix for deb_pkg_tools.deps.parse_relationship()
 - Added deb_pkg_tools.control.deb822_from_string() shortcut
 - Use assertEqual() instead of assertEquals() (deprecated apparently)
Monkey patch pprint so it knows how to `pretty print' RelationshipSet*

* very useful to verify docstrings containing doctest blocks