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

# Bug report

### Bug description:

```python
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