pytest is the preferred way to write and run unit tests these days and
it has a cleaner interface - so lets switch to it. The tokenizer tests
especially are much easier to read/understand.

This was mostly done with find/replace regexes and then cleaned up
manually.

Fixes #221.

See https://github.com/ogrisel/python-appveyor-demo/commit/09a1c8672e

* Remove unnecessary win_install.ps1.
* Add appveyor version update to release script.
* Make setup.py's pure Python fallback code less intrusive.