Remove pytest from setup requirements

[Singletoned]

It's not a great package to have in production, and doesn't actually seem to be needed for anything apart from running tests.

[liZe]

Hi!

Thank you for this pull request.

Pytest-runner is not Pytest, and it’s required in setup_requires if we want to have setup.py test work without installing pytest before.

Having pytest-runner in production is bad. Having pytest-runner is actually now bad everywhere, as it’s deprecated. But I didn’t find a clean solution yet to launch tests out of the box.

I’ll change CairoCFFI’s (and other projects’) packaging during the next months, and this change will include the removal of pytest-runner.