Migrate from Pipenv to Poetry

tox.ini

@@ -1,12 +1,9 @@
 [tox]
-envlist = py34, py35, py36, py37
-changedir=assets/onions/
-setupdir=assets/onions/
-skip_missing_interpreters = true
+isolated_build = true
+envlist = py36, py37
 
 [testenv]
-deps=
-    pytest
-    pyfakefs
-    pytest-mock
-commands=pytest -v
+whitelist_externals = poetry
+commands =
+    poetry install -v
+    poetry run pytest tests/