YDA-6066: Adjust OpenAPI script for Py3

Update generate-openapi script so that it works
correctly with Python 3.

This includes::
- Extracting functions to make code easier to understand.
- Instead of loading the ruleset and using custom instrumentation to
  inspect it, collect properties of API rules using the built-in
  AST module. This removes the need to update the OpenAPI script as
  dependencies change, since we no longer need to ensure we have stub
  code in place to be able to run the instrumentation.