`str-format` for fill and align specifiers

Feature

The fill and align string format specifiers do not currently have type checking, but are not supported by all types. Specifically, None does not support them.

Pitch

I ran into a bug where an optional type was being passed to string formatting using <. This would succeed most of the time, but error if the value was None:

>>> "{:<2}".format(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to NoneType.__format__

It would be great if mypy would catch this code as a type violation.

I attempted to implement here: https://github.com/python/mypy/pull/18799 but this was more challenging than I anticipated and I am a first time contributor.

Thanks.

まず PR #18799 で試みられた実装をレビューし、次に、fill および align 指定子に対する format-specifier の型チェックを mypy がどのように処理しているかを追跡します。完了条件は、None のようなオプショナル値がサポートされていない fill または align 指定子でフォーマットされたときに、mypy がエラーを報告することです。