Test failure in PSF/PRF photometry with latest Numpy

```
_________________________ ERROR collecting photutils/tests/test_psf_photometry.py __________________________
photutils/tests/test_psf_photometry.py:34: in <module>
>   image = np.zeros((image_size, image_size))
E   DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
```

This is with the latest Numpy dev version.