add a simple tox file

tox.ini

+[tox]
+envlist=flake8,py3
+
+[testenv:py3]
+deps =
+  pytest-cov
+commands =
+  pytest --cov=swh --cov-branch {posargs}
+
+[testenv:flake8]
+skip_install = true
+deps =
+  flake8
+commands =
+  {envpython} -m flake8