[Python] Bug Fix - model_generic templates to have valid imports for polymorphism

[YousefHaggy]

Fixes #20272

Can validate using the repro spec provided in the issue.

@cbornet @tomplus @krjakbrjak @fa0311 @multani

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[YousefHaggy]

@wing328 Could you take a look at this PR? #20272 results in a completely broken generated client. Thanks!