`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.

Esamina innanzitutto l’implementazione tentata in PR #18799, quindi traccia il modo in cui mypy gestisce il controllo dei tipi degli specificatori di formato per gli specificatori di riempimento e allineamento. Il lavoro è completato quando mypy segnala un errore se un valore opzionale come None viene formattato con uno specificatore di riempimento o allineamento non supportato.