We stopped caring about Python 2 support long time ago and with
libblockdev 3.0 which is Python 3 only it wouldn't even be
possible to support Python 2 so we should finally remove it.

These are needed only for the Pytho 2/3 compatibility and we no
longer support Python 2. unicodeize can be removed without
replacement and the default str constructor can replace stringize.

We no longer support older versions of Python so we can import
everything only from the unittest module.