[python] change Private attr to Class vars (#16687)

* [python] fix TypeError

Signed-off-by: ふぁ <yuki@yuki0311.com>

* [python] update Private model attributes to Class vars

Signed-off-by: ふぁ <yuki@yuki0311.com>

* [python] rename the List of test cases to ListClass

Signed-off-by: ふぁ <yuki@yuki0311.com>

* [python] update samples

Signed-off-by: ふぁ <yuki@yuki0311.com>

* [python] rename the List of v1 test cases to ListClass

Signed-off-by: ふぁ <yuki@yuki0311.com>

* [python] rename the List of v1-aiohttp test cases to ListClass

Signed-off-by: ふぁ <yuki@yuki0311.com>

* update samples

---------

Signed-off-by: ふぁ <yuki@yuki0311.com>
Co-authored-by: William Cheng <wing328hk@gmail.com>

samples/openapi3/client/petstore/python-pydantic-v1-aiohttp/test/test_list_class.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.list_class import ListClass  # noqa: E501
+
+class TestListClass(unittest.TestCase):
+    """ListClass unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def make_instance(self, include_optional) -> ListClass:
+        """Test ListClass
+            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 `ListClass`
+        """
+        model = ListClass()  # noqa: E501
+        if include_optional:
+            return ListClass(
+                var_123_list = ''
+            )
+        else:
+            return ListClass(
+        )
+        """
+
+    def testListClass(self):
+        """Test ListClass"""
+        # inst_req_only = self.make_instance(include_optional=False)
+        # inst_req_and_optional = self.make_instance(include_optional=True)
+
+if __name__ == '__main__':
+    unittest.main()