setup.py: Relax opencv dependency to 3.* (but still >= 3.2)
setup.py: Relax opencv dependency to 3.* (but still >= 3.2)

PyPI doesn't have 3.2 packages for MacOS 10.14, which was released 1
year after the most recent 3.2.x release (3.2.0.8 from July 2017).

I'll let opencv pull in the version of numpy that it needs -- in case
there are conflicts. I expect numpy to be fairly stable.

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