Finish develop to master automated pull requests

[bahmutov]

Based on the example in https://github.com/bahmutov/auto-pr-gh-action-example need to finish the setup to automatically propagate merged pull requests from develop to master.

• every merged pull request into develop should trigger action to open a pull request into master (or update existing one)
• the new pull request should have a specific label
• then install https://mergify.io/ app (free for public repos) and configure it to merge the pull requests with that specific label after the tests finish