Weak type checking mode

[JukkaL]

This is place-holder issue for a 'weak' type checking mode where mypy would do some useful type checking within functions that have no annotations.

[JukkaL]

We could use the same mode for module top levels by default to generate fewer errors.

This is a generalization of #374.

[JukkaL]

To rephrase, the objective is to do useful type checking without many false positives / useless reported errors for plain vanilla Python code without annotations.

[JelleZijlstra]

Weak mode was removed in #2213 last year, so I think this issue can be closed.