[Python] fixes __setitem__ throwing an error for composed instance

Created by: vvb

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Fixes #10083 (closed)

Test is also added.

Here is a snapshot of the test failing before adding the fix,

Here is a snapshot of the tests passing after adding the fix,

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) @wing328