[Python-experimental] fix name of generated python class for composed schema

Created by: sebastien-rosset

In some cases there is a mismatch in the name of the generated Python class. The python-experimental/model_templates/model_composed.mustache template is used to generate a Python class when the corresponding OAS model is a composed schema, i.e. oneOf, anyOf, allAll. The name of the class uses the {{unescapedDescription}} tag, whereas the composed elements used {{.}}, which may not yield the same results.

The name of the class may not match the name of the referenced class in the generated _composed_schemas() function.

Note: The use of "unescapedDescription" as a holder for the name of the Python class is very confusing. I saw the comments indicating there are no other fields that can be used, but still imo it's not a pretty workaround. https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientExperimentalCodegen.java#L753

The problem occurs in the following scenario:

components:
  schemas:
   os.Install:
      allOf:
        - '#/components/schemas/os.AbstractXYZ'
        - type: object
           properties:
               ...

The name of the generated class is "OsIntall", but the code generator also creates a reference to the "OSInstall" type.

Update: the PR as is does not work. @spacether , do you have a suggestion on how to change the template such that the compose schema references will match the unescapedDescription tag?

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.