[Python] Adds allOf/oneOf/anyOf composed models (#4446)

* Adds composed model support for allOf oneOf anyOf

* Moves discriminator into discriminator() class method, adds test_deserialize_lizard, adds setting discriminator using allOf schema

samples/client/petstore/python-experimental/test/test_parent.py

@@ -0,0 +1,37 @@
+# coding: utf-8
+
+"""
+    OpenAPI Petstore
+
+    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by: https://openapi-generator.tech
+"""
+
+
+from __future__ import absolute_import
+
+import unittest
+
+import petstore_api
+
+
+class TestParent(unittest.TestCase):
+    """Parent unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def testParent(self):
+        """Test Parent"""
+        # FIXME: construct object with mandatory attributes with example values
+        # model = petstore_api.Parent()  # noqa: E501
+        pass
+
+
+if __name__ == '__main__':
+    unittest.main()