[python] Fix circular imports on AllOf generation

[zishunwei]

Fix issue: #18271

When enabling REFACTOR_ALLOF_WITH_PROPERTIES_ONLY=True for a AllOf generation, the circular imports error occurs if two schemas refer to each other (like #/components/schemas/CircularAllOfRef and #/components/schemas/SecondCircularAllOfRef in spec added in test https://github.com/OpenAPITools/openapi-generator/pull/18272/files#diff-4f932bfd632d4afa271a4f3cc4d10554b4d08e8767b84aa77e50cc90c699ab3cR2709)

The root cause is it adds the import statement for the models in allVars and will be generated at the top of model file, the allVars will include all models referred in properties if enabling REFACTOR_ALLOF_WITH_PROPERTIES_ONLY=True https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java#L934, causing the circular imports error.

Since the import statements for these models are expected to be put in postponedModelImports and used by the mustache template, we can fix it by removing the items of postponedModelImports in modelImports if existed. This fix will not impacts on other cases

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@cbornet (2017/09) @tomplus (2018/10) @krjakbrjak (2023/02) @fa0311 (2023/10) @multani (2023/10)

[zishunwei]

Hi @fa0311 , could you please have a review if you've time since it might be based on your fix for Circular imports on inherited discriminators #17886 before, thanks a lot! cc: @wing328

[wing328]

lgtm. thanks for the fix

[zishunwei]

Hi @wing328 , looks my commits are not counted in https://github.com/OpenAPITools/openapi-generator/graphs/contributors although I have 2 PRs merged, any ideas? thanks

[wing328]

you need to have at least 15 commits in order to be listed in https://github.com/OpenAPITools/openapi-generator/graphs/contributors