add my.demo for testing out various approaches to configuring

tests/demo.py

@@ -0,0 +1,29 @@
+from pathlib import Path
+from more_itertools import ilen
+
+# TODO NOTE: this wouldn't work because of an early my.config.demo import
+# from my.demo import items
+
+def test_dynamic_config(tmp_path: Path) -> None:
+    import my.config
+
+    class user_config:
+        username  = 'user'
+        data_path = f'{tmp_path}/*.json'
+    my.config.demo = user_config # type: ignore[misc, assignment]
+
+    from my.demo import items
+    [item1, item2] = items()
+    assert item1.username == 'user'
+
+
+import pytest # type: ignore
+@pytest.fixture(autouse=True)
+def prepare(tmp_path: Path):
+    (tmp_path / 'data.json').write_text('''
+[
+    {"key1": 1},
+    {"key2": 2}
+]
+''')
+    yield