Don't include read-only properties in Python examples. (#9252)

* Don't include read-only properties in Python examples.

When using a schema with read-only fields as API inputs, Python
generates examples for those: this excludes them.

* Fix tests

samples/openapi3/client/petstore/python/test/test_readonly.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.readonly import Readonly
+
+
+class TestReadonly(unittest.TestCase):
+    """Readonly unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def testReadonly(self):
+        """Test Readonly"""
+        # FIXME: construct object with mandatory attributes with example values
+        # model = Readonly()  # noqa: E501
+        pass
+
+
+if __name__ == '__main__':
+    unittest.main()