Fix test for test_field_metadata_custom_mapping about rustpython HOT 1 CLOSED

Looks like there is something strange:

The dataclass docs state that the object given to metadata will be wrapped in a MappingProxyType()
https://docs.python.org/3/library/dataclasses.html#dataclasses.field

Following the link, it states that indexing with a key on MappingProxyType throws a KeyError.

But CPython produces an AttributeError and CPython's unit tests expect an AttributeError:
https://github.com/python/cpython/blob/5f4c7cf3f49629bcf890c305e0a90ffd708110a2/Lib/test/test_dataclasses/__init__.py#L2065C1-L2066C39

So it looks like that the documentation is partially wrong - MappingProxyType throws whatever the underlying structure throws, not always a KeyError as the documentation leads to believe.

from rustpython.