[BUG][Python-Flask] Primitive type bytearray deserialization missing in util.py

Created by: Shogoki

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• [X ] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

This PR fixes the issue #2724 (closed) using the suggested fix from @dannmartens adding "bytearray" to the template for util.py