[Python] remove default content-type in the request

Created by: kevchentw

Before this fix, the content-type of the request defaults to "application/json".

With this fix, we no longer set such default.

fix #8116 (closed), relate to https://github.com/OpenAPITools/openapi-generator/pull/10769/

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cc @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @Jyhess (2019/01) @arun-nalla (2019/11) @spacether (2019/11)

changed title from [Pytonn] remove default content-type in the request to [Python] remove default content-type in the request

mentioned in merge request !10783 (closed)

added Client: Python label

changed milestone to %5.3.1

changed milestone to %5.4.0

removed milestone

added Issue: Bug label

removed milestone

changed milestone to %6.0.0

closed