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

.travis.yml

@@ -44,6 +44,8 @@ jobs:
       env: TOXENV=py39-pytestlatest
     - python: "3.8"
       env: TOXENV=py38-pytestmaster
+    - python: "3.8"
+      env: TOXENV=py38-psutil
 
     - stage: deploy
       python: '3.8'