Update pytest-xdist to 1.16.0

[pyup-bot]

There's a new version of pytest-xdist available.
You are currently using 1.15.0. I have updated it to 1.16.0

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

Changelog

1.16.0

---

• pytest-xdist now requires pytest 2.7 or later.

• Add worker_id attribute in the TestReport

• new hook: pytest_xdist_make_scheduler(config, log), can return custom tests items
  distribution logic implementation. You can take a look at built-in LoadScheduling
  and EachScheduling implementations. Note that required scheduler class public
  API may change in next pytest-xdist versions.

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 #71