Add python-opencv as a dependency for ArchLinux

[jinnko]

In the AUR package details GJRodenburg noted the following:

Opencv has split of it's python parts since opencv 4.5.2-2, they are now available as python-opencv. This will need to be included as a dependency of Howdy.

Confirmed locally following an update that howdy was failing with:

Traceback (most recent call last):
  File "/lib/security/howdy/compare.py", line 18, in <module>
    import cv2
ModuleNotFoundError: No module named 'cv2'
Unknown error: 1

Following installation of python-opencv howdy worked as expected.

[boltgolt]

Thanks!