Fix indexing of eigen vector

[unnonouno]

from_rotation_matrix causes deprecation warning. It is because linalg.eigh returns a 2D ndarray. I fixed its indexing.

The simplest code to check is below:

import warnings
import numpy as np
import quaternion

warnings.simplefilter("default")                                                                                                                                                                                   
quaternion.from_rotation_matrix(np.eye(3))

And the warning message:

/home/unno/git/quaternion/src/quaternion/__init__.py:357: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
  q.components[0] = eigvecs[-1]

[moble]

Thanks very much!