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.

ジェネリックな foo と str.format の例を使って、テストケース testFormatCallGenerics の動作を再現します。mypy による str.format 呼び出しのチェックと外側のコンテキストの扱いを追跡します。完了条件は、有効な float のフォーマット呼び出しで誤検出が発生しなくなり、意図したフォーマット指定子のエラーが引き続きカバーされることです。