Make html2text.py compatible with python 3.*

* Don't expect u'' strings to work.
* Fix nbsp_unicode test.
* Also remove workaround for python < 2.3 ... we really don’t need it
  anymore.
* Fix <pre> in list tests.
  Replace onlywhite function with RE.

  I believe that Aaron by mistake put to the html2text.py his
  contribution to the Obfuscated Python Code Contest.

  For the reasons I have no chance to decipher, I have to use RE.
  More obvious replacement with para.isspace() doesn't work.

Signed-off-by: Matěj Cepl <mcepl@redhat.com>