Infer `"x" % ...` as being a string

[JelleZijlstra]

We already do this for str.format: https://github.com/JelleZijlstra/autotyping/blob/master/autotyping/autotyping.py#L536

[JelleZijlstra]

Similarly, we can infer that the following always return a bool:

• UnaryOp with not
• BinOp with in, not in, is, is not. (But not == which can return an arbitrary object.)