Improve error message when using `TypeVar` in functions

We're trying to add static typing support to the Application object in aiohttp. Currently the latest proposal is to define a custom key which can be used something like:

k = AppKey("k", int)
app[k] = "foo"

The method definition looks like:

def __setitem__(self, key: AppKey[_T], value: _T) -> None:

The problem is, that example produces a rather uninformative error:

Cannot infer type argument 1 of "__setitem__" of "Application"  [misc]

I'd really like to see something along the lines of "str" is not compatible with "int" so the user can easily understand the issue.

Related PR: https://github.com/aio-libs/aiohttp/pull/6498

Reproduce el ejemplo usando Application.setitem, AppKey y TypeVar; después, sigue la inferencia de tipos de funciones genéricas y el manejo de diagnósticos de mypy para el error [misc] indicado. Se considera terminado cuando la misma incompatibilidad produce un mensaje accionable como "str" is not compatible with "int", con el comportamiento relevante cubierto por pruebas de mypy.