add openapi-generator v5.x support to python client

[itaru2622]

related to #207,
this PR makes python client ready to openapi-generator v5.x.

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[itaru2622]

/assign @yliaog

[yliaog]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: itaru2622, yliaog

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[roycaihw]

I think it breaks the existing client generation:

org.apache.maven.plugin.MojoExecutionException: Illegal configuration: 'language' and  'generatorName' can not be set both, remove 'language' from your configuration
...
[ERROR] Failed to execute goal org.openapitools:openapi-generator-maven-plugin:4.3.0:generate (default) on project client-python: Code generation failed. See above for the full exception. -> [Help 1]