Fix serialization of inline arrays of free-form objects [python]

Created by: jsok

Given a spec like:

        "type": "array",
        "items": {
          "type": "object",
          "additionalProperties": true
        }

the generated code is unable to serialize it (reproduction in commit 834b4e3d):

test_model_to_dict (tests_manual.test_model_utils.ModelUtilsTests) ... ERROR

======================================================================
ERROR: test_model_to_dict (tests_manual.test_model_utils.ModelUtilsTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jsok/dev/github.com/OpenAPITools/openapi-generator/samples/openapi3/client/petstore/python/tests_manual/test_model_utils.py", line 32, in test_model_to_dict
    model_to_dict(model)
  File "/Users/jsok/dev/github.com/OpenAPITools/openapi-generator/samples/openapi3/client/petstore/python/petstore_api/model_utils.py", line 1690, in model_to_dict
    res.append(model_to_dict(v, serialize=serialize))
  File "/Users/jsok/dev/github.com/OpenAPITools/openapi-generator/samples/openapi3/client/petstore/python/petstore_api/model_utils.py", line 1662, in model_to_dict
    if model_instance._composed_schemas:
AttributeError: 'dict' object has no attribute '_composed_schemas'

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)

Subsequent commit 83327324 contains the fix in model_to_dict.

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@spacether related to our earlier discussion: https://github.com/OpenAPITools/openapi-generator/pull/9153/files#r712601002 @arun-nalla @Jyhess @tomplus