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.

Per http://docs.travis-ci.com/user/migrating-from-legacy

'nightly' now follows Python 3.6
Use '3.5-dev' for Python 3.5 nightly
http://docs.travis-ci.com/user/languages/python/#On-demand-installations

Partially resolves issue #112

'setup.py test' also uses SystemExit, with args[0] as False.
Detect and re-raise.

Add support for building without extension even when compiler
is functional, and set up extension-less travis builds.