fix(tests): add pyproject.toml so pytest picks up app.py from root

Without this, plain 'pytest' (as opposed to 'python -m pytest') fails
to import app.py because the repo root isn't on sys.path. CI hit this
on the first run.

Author: zsmithc2c

pyproject.toml

@@ -0,0 +1,3 @@
+[tool.pytest.ini_options]
+# Make `pytest` (without `python -m`) able to import app.py from the repo root.
+pythonpath = ["."]