It's EOL and AppVeyor builds are broken under it if we want to support Python 3.9.

Not fully deprecating 3.5 for now because it's still used on Toolforge.

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