[Python] Allow models to have properties of type self (#4888)

* Refactors openapi_types into a staticmethod

* Adds a Player model with a self type rpoperty, adds a test to ensure that it is working right

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

@@ -0,0 +1,43 @@
+# 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 TestPlayer(unittest.TestCase):
+    """Player unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def testPlayer(self):
+        """Test Player"""
+        # we can make a player without an enemy_player property
+        jane = petstore_api.Player(name="Jane")
+        # we can make a player with an enemy_player
+        sally = petstore_api.Player(name="Sally", enemy_player=jane)
+        # we can make a player with an inline enemy_player
+        jim = petstore_api.Player(
+            name="Jim",
+            enemy_player=petstore_api.Player(name="Sam")
+        )
+
+
+if __name__ == '__main__':
+    unittest.main()