Use of six.text type in isinstance

[marcdm]

As reported in #63, there are a couple places where isinstance(varname, six.text_type) is called. This is ok on Python 3, but not on Python 2.

This patch fixes the issue. I don't quite understand the format of the included test architecture, so I created a standalone test; included.

All 26941 tests pass on Python 2.7.5+ and 3.2.4 on Debian Jessie.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/179

This is an external review system which you may optionally use for the code review of your pull request.

[gsnedders]

There are review comments on Critic (I don't think you get notification emails till you first visit).

[marcdm]

The intention with the test is to simulate a naive user using Python 2.x and casually adding an attribute without specifying strings as u'att_name' or importing unicode_literals.

The tests run by test_treewalker_six_mix will pass on Python 3.x but fail on Python 2.x and the pull-request contains a fix and some suggestions for minor code refactoring ( e.g. to_text and is_text_or_none )

[gsnedders]

Merged in 1d3434f.