Add coverage to ensure the public API remains usable (#23)

tests/test_api.py

@@ -0,0 +1,11 @@
+"""Test we do not break the public API."""
+
+from propcache import _helpers, api
+
+
+def test_api():
+    """Verify the public API is accessible."""
+    assert api.cached_property is not None
+    assert api.under_cached_property is not None
+    assert api.cached_property is _helpers.cached_property
+    assert api.under_cached_property is _helpers.under_cached_property