tox: add a simple tox file that runs tests, flake8 and check-manifest

note that unit tests are executed via pifpaf, so a running postgres
server is not required (nor used) when unit tests are spawn with tox.

Also add .tox in gitignore.

.gitignore

@@ -7,3 +7,4 @@ __pycache__
 dist
 swh.core.egg-info
 version.txt
+.tox

tox.ini

+[tox]
+envlist=check-manifest,flake8,py3
+
+[testenv:py3]
+deps =
+  pifpaf
+  nose
+commands =
+  pifpaf run postgresql -- nosetests
+
+[testenv:flake8]
+skip_install = true
+deps =
+  flake8
+commands =
+  {envpython} -m flake8
+
+[testenv:check-manifest]
+skip_install = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest {toxinidir}