fix #11958 [BUG] python generates wrong model name and model file name (#11959)

* fix #11958 [BUG] python generates wrong model name and model file name

Modify AbstractPythonCodegen.toModelName just like AbstractJavaCodegen.toModelName

* add unit test

* update samples and docs
by
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh

* fix AbstractPythonCodegen#toModelName logic, remove underscore

* update samples and docs
by
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh

samples/client/petstore/python/test/test_model_200_response.py

@@ -0,0 +1,35 @@
+"""
+    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
+"""
+
+
+import sys
+import unittest
+
+import petstore_api
+from petstore_api.model.model_200_response import Model_200Response
+
+
+class TestModel_200Response(unittest.TestCase):
+    """Model_200Response unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def testModel_200Response(self):
+        """Test Model_200Response"""
+        # FIXME: construct object with mandatory attributes with example values
+        # model = Model_200Response()  # noqa: E501
+        pass
+
+
+if __name__ == '__main__':
+    unittest.main()