Assertion rewrite should match files completely and avoid rewriting setup.py

[Quoting](https://github.com/pypa/setuptools/issues/1170#issuecomment-346174810) @hroncok in pypa/setuptools#1170:

> * _pytest.config._mark_plugins_for_rewrite should not register setup when it sees setup.py
> * _pytest.assertion.rewrite._should_rewrite should probably match things that startswith mark + '.', not just mark (although I'm not very sure here)