[REQ] Python .gitlab-ci.yml

Created by: LoveIsGrief

Is your feature request related to a problem? Please describe.

It would be great to have a .gitlab-ci.yml for the python package. It would support Gitlab CI and not only TravisCI.

Describe the solution you'd like

A simple .gitlab-ci.yml for python that does the same as .travis.yml.

Describe alternatives you've considered

A simpler way to include files in templates? I'm not sure one exists and if it does, it has to be documented.

Additional context

I'm talking about adding .gitlab-ci.mustache to https://github.com/OpenAPITools/openapi-generator/tree/ea55968737fa80a3e306d1b08032ff4d911774c9/modules/openapi-generator/src/main/resources/python