[PYTHON] variable substitution not occurring during template generation OpenAPI spec 3.0

Description

I'm using server templating in my swagger.yaml and swagger-codegen-cli-3.0.21 JAR file to generate a Python SDK. In the generated configuration.py file, the host is populated as a string with the placeholders rather than being substituted. This consequently returns "name or service unknown" error when the client tries to connect to the host

Swagger-codegen version

swagger-codegen-cli-3.0.21

Swagger declaration file content or url

openapi: 3.0.0
info:
  title: my model
  version: '1.0'
servers:
- url: '{protocol}://{server}/{basePath}'
  variables:
    protocol:
      enum:
      - http
      - https
      default: http
    server:
      enum:
      - localhost:1234
      - prod_server
      default: localhost:1234
    basePath:
      default: v1/mymodel

Command line used for generation

java -jar swagger-codegen-cli-3.0.21 generate -i myclient/swagger.yaml -o myclient/ -l python

Steps to reproduce

run command line above, returns configuration.py with:

self.host = "{protocol}://{server}/{basePath}"

Related issues/PRs

Suggest a fix/enhancement

have codegen perform variable substitution on the default values and populate host accordingly in configuration.py. Similar functionality has been implemented in OpenAPI generator which handles both multiple servers and variable substitution

Reproduce the issue with the provided OpenAPI YAML and the swagger-codegen-cli-3.0.21 command, then inspect the generated configuration.py where the unresolved host is assigned. Compare the requested behavior with the referenced OpenAPI generator implementation. Done means default server variables are substituted in the generated Python client so the host is usable.