Add a deprecation warning for the "objects" property of

observed-data.  Add a unit test to ensure we get the warning.

stix2/test/v21/test_observed_data.py

@@ -1375,3 +1375,18 @@ def test_new_version_with_related_objects():
     new_version = data.new_version(last_observed="2017-12-12T12:00:00Z")
     assert new_version.last_observed.year == 2017
     assert new_version.objects['domain'].resolves_to_refs[0] == 'src_ip'
+
+
+def test_objects_deprecation():
+    with pytest.deprecated_call():
+        stix2.v21.ObservedData(
+            first_observed="2016-03-12T12:00:00Z",
+            last_observed="2016-03-12T12:00:00Z",
+            number_observed=1,
+            objects={
+                "0": {
+                    "type": "file",
+                    "name": "foo",
+                },
+            },
+        )

stix2/v21/sdo.py

@@ -2,6 +2,7 @@
 
 from collections import OrderedDict
 import itertools
+import warnings
 
 from six.moves.urllib.parse import quote_plus
 
@@ -573,6 +574,13 @@ class ObservedData(STIXDomainObject):
         self.__allow_custom = kwargs.get('allow_custom', False)
         self._properties['objects'].allow_custom = kwargs.get('allow_custom', False)
 
+        if "objects" in kwargs:
+            warnings.warn(
+                "The 'objects' property of observed-data is deprecated in "
+                "STIX 2.1.",
+                DeprecationWarning,
+            )
+
         super(ObservedData, self).__init__(*args, **kwargs)
 
     def _check_object_constraints(self):