Make tox -p auto handle coverage right

BTW I had some problems using tox -p auto before I started passing
--parallel-safe-build.  I'm not sure why that isn't on by default.

tox.ini

@@ -1,6 +1,8 @@
 [tox]
 envlist =
     py27,py27-pure,py35,py35-pure,py36,py37,pypy,pypy3,coverage,docs
+# NB: if you add new environments, please also add them to depends in
+# testenv:coverage so that parallel runs (tox -p auto) will work correctly
 
 [testenv]
 commands =
@@ -36,6 +38,8 @@ commands =
     coverage report
     coverage html
     coverage xml
+depends = py27,py27-pure,py35,py35-pure,py36,py37,pypy,pypy3
+parallel_show_output = true
 
 [testenv:docs]
 basepython =