Type hints for `Table.update` are inaccurate?

[brendan-m-murphy]

The type hints for Table.update say that if fields is Callable, then it must map Mapping to None. This probably either needs to be Callable[[MutableMapping], None] or Callable[[Mapping], Mapping].

For instance, the delete operation creates a "transform" function that deletes a key. But

def transform(doc: Mapping) -> None:
    del doc[field]

is invalid since Mapping doesn't have __delitem__: https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes.

So either doc should be MutableMapping, or the operation should return the updated doc:

def transform(doc: Mapping) -> dict:
    doc = dict(doc)
    del doc[field]
    return doc