add tests for deprecation warnings

tests/test_models.py

@@ -351,3 +351,18 @@ def find_gen_by_id(object_id):
             # many models which would indicate they are difficult to garbage
             # collect.
             assert len(mids) < 2
+
+
+def test_read_deprecation(tmp_path):
+    fn = tmp_path / "test.fits"
+    m = DataModel()
+    m.save(fn)
+    with pytest.warns(DeprecationWarning, match="read is deprecated"):
+        m.read(fn)
+
+
+def test_write_deprecation(tmp_path):
+    fn = tmp_path / "test.fits"
+    m = DataModel()
+    with pytest.warns(DeprecationWarning, match="write is deprecated"):
+        m.write(fn)