Better messages when type is incompatible due to invariance

[JukkaL]

Users are often confused by compatibility of invariant collection types. Here is an example from #3351:

from typing import List, Union

def mean(numbers: List[Union[int, float]]) -> float:
    return sum(numbers) / len(numbers)

some_numbers = [1, 2, 3, 4]
mean(some_numbers)  # Argument 1 to "mean" has incompatible type List[int]; 
                    # expected List[Union[int, float]]

It might be helpful to add a note with more details, since the message doesn't really provide enough context to start googling for help. Here is one idea:

program.py:23:note: "List" is invariant -- see <link to docs>
program.py:23:note: Maybe you can use "Sequence" as the target type, which is covariant?

[ilevkivskyi]

Isn't this a duplicate of #1115?

[quartox]

I will start looking at this.

[ilevkivskyi]

This is now fixed by #3411