stbt lint: Whitelist `imwrite` from OpenCV 3.x
stbt lint: Whitelist `imwrite` from OpenCV 3.x

This fixes
`test_that_stbt_lint_ignores_images_created_by_the_stbt_script` on
Ubuntu 18.04.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester