`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.

首先审查 PR #18799 中尝试的实现，然后追踪 mypy 对填充和对齐说明符的格式说明符类型检查的处理方式。完成的标准是：当使用不受支持的填充或对齐说明符格式化 None 等可选值时，mypy 会报告错误。