use model class for request body with additionalProperties (#16613)

* try out simple fix

* add test case

* generate samples

samples/openapi3/client/petstore/python/test/test_test_inline_freeform_additional_properties_request.py

@@ -0,0 +1,52 @@
+# 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: \" \\
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by OpenAPI Generator (https://openapi-generator.tech)
+
+    Do not edit the class manually.
+"""  # noqa: E501
+
+
+import unittest
+import datetime
+
+from petstore_api.models.test_inline_freeform_additional_properties_request import TestInlineFreeformAdditionalPropertiesRequest  # noqa: E501
+
+class TestTestInlineFreeformAdditionalPropertiesRequest(unittest.TestCase):
+    """TestInlineFreeformAdditionalPropertiesRequest unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def make_instance(self, include_optional) -> TestInlineFreeformAdditionalPropertiesRequest:
+        """Test TestInlineFreeformAdditionalPropertiesRequest
+            include_option is a boolean, when False only required
+            params are included, when True both required and
+            optional params are included """
+        # uncomment below to create an instance of `TestInlineFreeformAdditionalPropertiesRequest`
+        """
+        model = TestInlineFreeformAdditionalPropertiesRequest()  # noqa: E501
+        if include_optional:
+            return TestInlineFreeformAdditionalPropertiesRequest(
+                some_property = ''
+            )
+        else:
+            return TestInlineFreeformAdditionalPropertiesRequest(
+        )
+        """
+
+    def testTestInlineFreeformAdditionalPropertiesRequest(self):
+        """Test TestInlineFreeformAdditionalPropertiesRequest"""
+        # inst_req_only = self.make_instance(include_optional=False)
+        # inst_req_and_optional = self.make_instance(include_optional=True)
+
+if __name__ == '__main__':
+    unittest.main()