The OpenAPI2.0 schema is built on top of the schema published by Cisco's Devnet here
The dnac1.2_swagger_custom.yml is used to add new path and model definitions that are not currently included in the published schema. The two two schema are merged and then the python package dnac-api-client is built using swagger_codegen (see build_swagger_client.sh)
Note: During the merge, we also add a security definition and modify the path /api/system/v1/auth/token to use the basic method. This makes getting a token using the dnac_api_client package a little easier (see examples).
The resulting python package is maintained on github here.
Please feel free to fork this project and make a pull request for any updates or modifications to the dnac1.2_swagger_custom.yml so that they can be included in dnac-api-client package.