Python frontend: async functions

[maximiliankaul]

Decide how to handle the async Python stuff. Currently, this is mostly "not supported" and yields a ProblemNode.
Ideas:

• these are basically the same as the non-async stuff -> make them inherit from the non-async classes and use the same functions in the handlers.
  • Pro: less duplicate code 👍
  • Con: this violates the Python AST grammar where they do not inherit form the non-async classes. I would like to have this reflected in our code (I only care about the Python.kt file here. If someone knows some kotlin magic to keep them separated there and still use the same implementations in the handlers 🥳 ).
• keep them separate -> duplicate code. We still need to implement (copy) the async stuff then.
• better ideas?

[maximiliankaul]

The duplicate code was removed with #1673

Remaining task:

• decide how to represent async in the graph and implement it.