Detecting whether a function has implicit `None` returns

[tusharsadhwani]

Basic control flow can be implemented to remove all false positive cases where implicit None return may happen.

The strategy

• visiting If, While and For nodes, to find all control flow paths inside a function definition.
• If any of the paths don't terminate with a return or raise (or something like exit() or sys.exit()), we assume that the function can return implicitly.
• Adding another flag, seen_implicit_none_return to state, and storing the information there.

If it sounds good, I can try and implement exactly this.

[JelleZijlstra]

I'm not enthusiastic about this because I want autotyping to remain a simple tool, not a type inference engine. I already help maintain two type checkers (pyanalyze and mypy) and that's plenty. Specifically the "or something like exit() or sys.exit()" information: doing that right requires us to look in typeshed or annotations to figure out which functions return Never.