`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.

Review the attempted implementation in PR #18799 first, then trace mypy's handling of format-specifier type checking for fill and align specifiers. Done means mypy reports an error when an optional value such as None is formatted with an unsupported fill or alignment specifier.