Discovered while adding MyPy for code that was implementing
MutableMapping and using the update function like this:
```python
class CaseInsensitiveDict(collections.MutableMapping):
def __init__(self, data=None, **kwargs):
# type: (dict, **Any) -> None
self._store = dict() # type: dict
if data is None:
data = {}
self.update(data, **kwargs)
```
This commit adds kwargs to MutableMapping to allow this.
Shout out to Tim Abbott for assisting me with this.
