Improve message if user forgot to call a function

[JukkaL]

Forgetting to call a method, i.e. missing (), is a somewhat common programming error. Mypy will typically catch this error, but often error messages includes something like "Callable[...]" has no attribute "foo". Since the callable type may be pretty complicated, this can look confusing.

Here are a few ideas that could make this better:

1. Only say Callable object has no attribute "foo" (omit the full callable type).
2. Add a note such as Did you mean ".foo()" (with parentheses)? (needs wordsmithing).
3. In addition to (1), show the signature of the function in a separate note pretty-printed using function definition syntax such as def foo() -> List[X].