`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.

Revisa primero la implementación intentada en PR #18799 y, después, sigue cómo mypy gestiona la comprobación de tipos de los especificadores de formato para los especificadores de relleno y alineación. La tarea estará terminada cuando mypy informe de un error al formatear un valor opcional como None con un especificador de relleno o alineación no compatible.