string.Formatter does not handle non-indexed item and attribute access

[dg-pb]

Bug report

Bug description:

from argparse import Namespace
fmt = '{[0]} {.a}'
fmt.format([10], Namespace(a=1))    # '10 1'

# ---

import string
string.Formatter().format(fmt, [10], Namespace(a=1))
# KeyError: ''

CPython versions tested on:

3.14

Operating systems tested on:

macOS

[dg-pb]

It is arguably a missing feature and not a bug, but I would say it is a bug given this statement in docs:
"The Formatter class in the string module allows you to create and customize your own string formatting behaviors using the same implementation as the built-in format() method."

[dg-pb]

Seems like a long-standing issue. Open duplicates:

• string.Formatter.parse does not handle auto-numbered positional fields #89867
• string.Formatter class not allowing {.field} #88662
• string.Formatter does not support key/attribute access on unnumbered fields #71494

[dg-pb]

#21767 closes all 3 above and this one.
It is ready for core review.