Fix test discovery

.vscode/settings.json

@@ -1,10 +1,5 @@
 {
     "python.pythonPath": ".venv/bin/python",
-    "python.analysis.extraPaths": [
-        "my_module",
-        "tests",
-        ".venv/bin/python"
-    ],
     // PEP8 settings (https://www.python.org/dev/peps/pep-0008/)
     "editor.tabSize": 4,
     "editor.insertSpaces": true,

tests/__init__.py

@@ -0,0 +1 @@
+"""Test cases."""