Use empty outer context when checking str.format() calls

This is a follow up for https://github.com/python/mypy/pull/7418

Because mypy is still too eager about outer context and str.format() has object type for its arguments, passing a call to generic function as an argument to str.format() may cause false positives. For example:

'{:.3f}'.format(sum(list_of_floats))

Fails with

Incompatible types in string interpolation (expression has type "object", placeholder has type "Union[int, float]")

A more rectified test case:

[case testFormatCallGenerics]
from typing import TypeVar, Union

T = TypeVar('T')
def foo(x: T) -> Union[T, int]: ...

x: str
y: float

"{:.3f}".format(foo(x))  # E: Incompatible types in string interpolation (expression has type "str", placeholder has type "Union[int, float]")
"{:.3f}".format(foo(y))
[builtins fixtures/primitives.pyi]

A potential simple fix is to re-accept the expression in empty context before checking the format specifier type.

Reproduce el comportamiento del caso de prueba testFormatCallGenerics usando los ejemplos genéricos de foo y str.format. Sigue la comprobación de llamadas a str.format de mypy y su gestión del contexto externo; terminado significa que la llamada válida de formateo de float ya no produce un falso positivo, mientras que el error previsto del especificador de formato sigue cubierto.