tox: Stop using 'python setup.py test'

pbr has deprecated this. Call stestr directly instead.

Change-Id: I26628e4734c6d7a911fbe16ad753173a36fd8dab
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -3,7 +3,6 @@ minversion = 2.0
 envlist = py27,py37,pep8
 
 [testenv]
-install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
  -r{toxinidir}/test-requirements.txt
@@ -35,7 +34,13 @@ commands =
 
 [testenv:cover]
 basepython = python3
-commands = python setup.py test --coverage --coverage-package-name=oslo_context --testr-args='{posargs}'
+setenv =
+  PYTHON=coverage run --source oslo_context --parallel-mode
+commands =
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.