test: add abstract cloudevent coverage tests

Signed-off-by: Alexander Tkachev <sasha64sasha@gmail.com>

cloudevents/tests/test_abstract_cloudevent.py

@@ -0,0 +1,41 @@
+from cloudevents.abstract import CloudEvent
+import pytest
+
+
+def test_del_is_abstract():
+    """
+    exists mainly for coverage reasons
+    """
+    with pytest.raises(TypeError):
+        del CloudEvent()["a"]
+
+
+def test_set_is_abstract():
+    """
+    exists mainly for coverage reasons
+    """
+    with pytest.raises(TypeError):
+        CloudEvent()["a"] = 2
+
+
+def test_create_is_abstract():
+    """
+    exists mainly for coverage reasons
+    """
+    assert CloudEvent.create({}, None) is None
+
+
+def test_data_read_is_abstract():
+    """
+    exists mainly for coverage reasons
+    """
+    with pytest.raises(TypeError):
+        CloudEvent()._data_read_model
+
+
+def test_attributes_read_model_is_abstract():
+    """
+    exists mainly for coverage reasons
+    """
+    with pytest.raises(TypeError):
+        CloudEvent()._attributes_read_model