+= operator fails for flex arrays when adding numpy-typed values

Example:
```
$ cat repro.py
import numpy as np
arr1 = np.array([1,2,3], dtype=np.float32)
from scitbx.array_family import flex
arr2 = flex.double(10)
arr2[0] += arr1[0]

$ libtbx.python repro.py
Traceback (most recent call last):
  File "/dev/shm/dwpaley/20250610/modules/repro.py", line 5, in <module>
    arr2[0] += arr1[0]
    ~~~~^^^
TypeError: 'int' object is not iterable

```