Update pytest-runner to 4.0

[pyup-bot]

There's a new version of pytest-runner available.
You are currently using 3.0. I have updated it to 4.0

These links might come in handy: PyPI | Changelog | Repo

Changelog

4.0

===

• Drop support for Setuptools before Setuptools 27.3.0.

3.0.1

=====

• 38: Fixed AttributeError when running with --dry-run.
  PyTest.run() no longer stores nor returns the result code.
  Based on the commit message for 840ff4c < https://github.com/pytest-dev/pytest-runner/commit/840ff4c2bf6c752d9770f0dd8d64a841060cf9bc>_,
  nothing has ever relied on that value.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #28