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 the behavior in the test case testFormatCallGenerics using the generic foo and str.format examples. Trace mypy's str.format call checking and its handling of outer context; done means the valid float formatting call no longer produces a false positive while the intended format-specifier error remains covered.