Support using either pymaging or PIL for tests

Instead of using Python version to determine whether pymaging is available or not, just try to import it. If it is not installed, try PIL as well. Then use either of them to run the PNG test or skip it appropriately.

This makes it possible to run the tests without having to install pymaging that lacks PyPi releases.

Additionally, I have added `six` dep to `tox.ini` since otherwise tests fail to run. You may want to remove either pillow or pymaging there since having both seems a bit pointless.