Migrate from testr to stestr

Note that the interface doesn't actually have tests.  This is just to
migrate away from the deprecated testr module.
Add .zuul.yaml so that the project can be merged

Change-Id: I0b8ba13297fe9f08e398045598f2b73e60facadd

.gitreview

@@ -0,0 +1,4 @@
+[gerrit]
+host=review.opendev.org
+port=29418
+project=openstack/charm-interface-keystone-notifications.git

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./unit_tests
+top_dir=./

.zuul.yaml

@@ -0,0 +1,3 @@
+- project:
+    templates:
+      - openstack-python3-charm-jobs

test-requirements.txt

@@ -1,2 +1,2 @@
-flake8>=2.2.4,<=2.4.1
-os-testr>=0.4.1
+flake8>=2.2.4
+stestr>=2.2.0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py34,py35
+envlist = pep8,py35
 skipsdist = True
 # NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
@@ -11,19 +11,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
   pip install {opts} {packages}
-commands = ostestr {posargs}
-
-[testenv:py27]
-basepython = python2.7
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-commands = /bin/true
-
-[testenv:py34]
-basepython = python3.4
-deps = -r{toxinidir}/test-requirements.txt
-# TODO: Need to write unit tests then remove the following command.
-commands = /bin/true
+commands = stestr run {posargs}
 
 [testenv:py35]
 basepython = python3.5
@@ -31,6 +19,24 @@ deps = -r{toxinidir}/test-requirements.txt
 # TODO: Need to write unit tests then remove the following command.
 commands = /bin/true
 
+[testenv:py36]
+basepython = python3.6
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+commands = /bin/true
+
+[testenv:py38]
+basepython = python3.8
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+commands = /bin/true
+
+[testenv:py3]
+basepython = python3
+deps = -r{toxinidir}/test-requirements.txt
+# TODO: Need to write unit tests then remove the following command.
+commands = /bin/true
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt