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

tox.ini

@@ -3,9 +3,9 @@ envlist=
   linting
   py{35,36,37,38,39}-pytestlatest
   py38-pytestmaster
+  py38-psutil
 
 [testenv]
-passenv = USER USERNAME
 extras = testing
 deps =
   pytestlatest: pytest
@@ -13,8 +13,16 @@ deps =
 commands=
   pytest {posargs}
 
+[testenv:py38-psutil]
+extras =
+  testing
+  psutil
+deps = pytest
+commands =
+  pytest {posargs:-k psutil}
+
 [testenv:linting]
-skipsdist = True
+skip_install = True
 usedevelop = True
 deps =
   pre-commit
@@ -28,9 +36,9 @@ skipsdist = True
 usedevelop = True
 passenv = *
 deps =
-    towncrier
+  towncrier
 commands =
-    towncrier --version {posargs} --yes
+  towncrier --version {posargs} --yes
 
 [pytest]
 addopts = -ra