Add tox for testing

pyproject.toml

@@ -54,6 +54,7 @@ pytest-mock = "^1.9"
 pygments-github-lexers = "^0.0.5"
 black = { version = "^18.3-alpha.0", python = "^3.6" }
 pre-commit = "^1.10"
+tox = "^3.0"
 
 
 [tool.poetry.scripts]

tox.ini

+[tox]
+skipsdist = True
+envlist = py27, py34, py35, py36, py37
+
+[testenv]
+whitelist_externals = poetry
+skip_install = true
+commands =
+    poetry install -v
+    poetry run pytest tests/