Warn about unannotated functions containing annotations

[gvanrossum]

I found some examples (https://bugs.python.org/issue28936) of functions that have no signature annotations but contain local variable annotations. These are currently ignored. I think we should actually warn whenever an annotation (either a type comment or PEP 526 style) occurs in an unannotated function, since it likely reflects some kind of misunderstanding about annotations.

Or at the very least we should have an option to warn about such things.

(IIRC we recently doused a similar source of confusion, but I can't recall the details.)

[ilevkivskyi]

Isn't this a duplicate of #3948? Or am I missing something?

[gvanrossum]

How quickly we forget. :-)