chore: configure tox for testing

Author: portswigger-tim

.github/workflows/docker-images.yml

@@ -0,0 +1,2 @@
+def test_always_passes():
+    assert True

ical-simplejson-proxy/test_app.py

@@ -0,0 +1,23 @@
+[tox]
+skipsdist=True
+envlist=py{38,39,310}, black
+
+[gh-actions]
+python =
+    3.8: py38
+    3.9: py39
+    3.10: py310
+
+[testenv]
+commands=py.test -vv {posargs:}
+deps=
+    pytest
+    pytest-cov
+    mock
+    pytest-mock
+    requests-mock
+
+[testenv:black]
+basepython = python3.9
+deps = black
+commands = black --check .