Add psutil extra and introduce pytest_xdist_auto_num_workers hook

This makes using psutil optional and opens up the possibility of customization
through the pytest_xdist_auto_num_workers hook, making things like #477 possible.

Fix #585

.appveyor.yml

@@ -5,6 +5,7 @@ environment:
   - TOXENV: "py37-pytestlatest"
   - TOXENV: "py38-pytestlatest"
   - TOXENV: "py38-pytestmaster"
+  - TOXENV: "py38-psutil"
 
 install:
   - C:\Python38\python -m pip install -U pip setuptools virtualenv