Investigate tooling to check CPython API use

[ronaldoussoren]

CPython API misuse can be hard to diagnose, especially reference counting. problems and forgetting to clear errors.

Investigate using (custom?) tooling to automate checking:

• Using annotations to document expected reference count behaviour, with tooling to verify this (compare how clang's static analyzer does this for ObjC retain counts)
• Use annotations to mark which functions can return with an active exception and use tooling to check error propagation/clearing.

With some luck someone has written tooling for this, otherwise the most complete fix is likely a plugin for the clang static analyzer (which is a lot of work...)