[python] asyncio supports _preload_content; remove unsed imports

[tomplus]

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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

[wing328]

Tests passed via https://travis-ci.org/OpenAPITools/openapi-generator/builds/383216868