`types.MappingProxyType` is documented as 'Read-only proxy of a mapping'.
But if used with a `collections.defaultdict` mapping, it can modify the underlying mapping.
```
import collections, types
dd = collections.defaultdict(set)
mpt = types.MappingProxyType(dd)
mpt['__getitem__'] # key inserted
mpt.get('get') # key not inserted
print(dd.items()) # dict_items([('__getitem__', set())])
```