[python] remove imports from models

[tomplus]

PR checklist

• [x] Read the contribution guidelines.
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Description of the PR

It resolves the issue https://github.com/swagger-api/swagger-codegen/issues/7541 (circular dependencies in Python models), based on @wy-z work, thanks.

PTAL @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11)

[kenjones-cisco]

LGTM! Thanks!

[wy-z]

+1