Add numpy __array__ interface

[mwtoews]

This adds a basic __array__ interface using the latest numpy docs. It does not require a numpy dependency.

Before this PR, attempting to create a NumPy array would only make a 1D array with 9 items:

>>> import numpy as np
>>> from affine import Affine
>>> tfm = Affine(*np.linspace(0.1, 0.6, 6))
>>> tfm
Affine(0.1, 0.2, 0.30000000000000004,
       0.4, 0.5, 0.6)
>>> np.array(tfm)
array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0. , 0. , 1. ])

now with this PR it is a 3x3 array as should be expected:

>>> np.array(tfm)
array([[0.1, 0.2, 0.3],
       [0.4, 0.5, 0.6],
       [0. , 0. , 1. ]])

[sgillies]

Thanks @mwtoews !