PR checklist

• Read the contribution guidelines.
• 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\.
• Filed the PR against the correct branch: Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Based on swagger's PRs:

• [python] remove imports from models swagger-api/swagger-codegen#7905
• [python-asyncio] client supports _preload_content swagger-api/swagger-codegen#8204

Resolves: swagger-api/swagger-codegen#7541

Thanks