[Python] Support _preload_content flag in python client

[mbohlool]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch 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)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

In order to support streaming responses, we need the API Client not to read response content and return a Response object. urllib3 has a flag called preload_content that if set to False, it will return a urllib3.HTTPResponse. This PR add support for this flag in rest client and api client and generated api classes. The flag is optional and defaulted to True to keep current behavior.

[wing328]

@mbohlool thanks for the contribution. If it's not too much to ask, do you mind adding a test case for _preload_content in https://github.com/swagger-api/swagger-codegen/blob/master/samples/client/petstore/python/tests/test_pet_api.py so that the test case won't pass in case some future changes break it?

cc @scottrw93

[mbohlool]

@wing328 done #4172