Default Values for TypedDict

It would be nice if you could set default values for a `TypedDict`:

```python
class Coordinate(TypedDict):
    x: int = 0
    y: int = 0
    z: int = 0

# current behavior
assert Coordinate() == {}
# desired behavior
assert Coordinate() == {"x": 0, "y": 0, "z": 0}
```