BitRound codec returns NaN if keepbits has dtype int32 instead of int64

This one took a while to track down...

Version: 0.15.1
Numpy: 2.2.6
Python 3.12.11

Also tested on 0.16.1 with the same results. 

Note that on a different machine with a different OS, I can't reproduce the bug. 

```
import numpy as np
from numcodecs.bitround import BitRound

codec = BitRound(keepbits=np.int32(10))
a = np.array([0.])
codec.decode(codec.encode(a))
```
yields
```
array([nan])
```
