Incremental crash caused by TypedDict deserialization

When type checking `t.py` twice using `-i`, the second run crashes:

```py
# t.py
from b import A
y: A
reveal_type(y)
```

```py
# b.py
from mypy_extensions import TypedDict
A = TypedDict('A', {'x': int, 'y': str})
```

I already have a fix in a local branch and I will send a PR soon.