Issue with longdouble in numpy

I have this in some code:
```
logarr = np.fromiter(loglist, np.longdouble, -1)
```
which causes problems when using the numpy stubs:
```
simple_model_kl.py:20: error: Module has no attribute "longdouble"
```
I think it's correct to just add
```
class longdouble(floating): ...
```
to `__init__.pyi`