What's the best way to use Numpy to convert a size (x, y, 3) array of rgb pixel values to a size (x, y, 1) array of grayscale pixel values?

I have a function, rgbToGrey(rgbArray) that can take the [r,g,b] array and return the greyscale value. I'd like to use it along with Numpy to shrink the 3rd dimension of my array from size 3 to size 1.

How can I do this?

Note: This would be pretty easy if I had the original image and could grayscale it first using Pillow, but I don't have it.

UPDATE:

The function I was looking for was np.dot().

From the answer to this quesiton:

Assuming we convert rgb into greyscale through the formula:

.3r * .6g * .1b = grey,

we can do np.dot(rgb[...,:3], [.3, .6, .1]) to get what I'm looking for, a 2d array of grey-only values.