Fix #7865: Incorrect handling of --model-name-prefix/suffix (python-f…

[univac490]

PR checklist

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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 and ./bin/security/{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.
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Description of the PR

Fix import in *_controller.py when using --model-name-prefix or --model-name-suffix

java -jar swagger-codegen-cli.jar generate -l python-flask --model-name-prefix swg

Before:

from swagger_server.models.swg_swg_error import SwgError

After:

from swagger_server.api_models.swg_error import SwgError