[Python] add the multipart files after sanitization in api_client

Created by: rienafairefr

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, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/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: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @wing328 @taxpon @frol @mbohlool @cbornet @kenjones-cisco

Description of the PR

When doing multipart file uploads, the passed file was parsed as a (fileName, fileContent, fileMIME) tuple, which was serialized to a string before being used, which is broken. With this PR, file contents are used in urllib3 etc with the correct format (a tuple (fileName, fileContent, fileMIME) ) Tested working on a real-worl API using multipart file upload for mixed binary/json files 👍