Add tox file

This is the defacto way to run unit tests for most Python projects. The
tox-poetry extension is used to provide integration with poetry.

Signed-off-by: Stephen Finucane <stephen@that.guru>

.github/workflows/python-test.yml

@@ -23,7 +23,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
-    
+
       - name: Get full Python version
         id: full-python-version
         run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")

tox.ini

@@ -0,0 +1,7 @@
+[tox]
+minversion = 3.18.0
+requires = tox-poetry
+
+[testenv]
+commands =
+  pytest {posargs:tests/}