Convert @unittest.skipIf / @unittest.skipUnless

[giampaolo]

Hello. I've been using this nice tool to convert psutil and pyftpdlib code bases. One of the few things that are left out from the conversion are @unittest.skipIf and @unittest.skipUnless decorators, which can be converted to @pytest.mark.skipif(condition, reason="reason"). It would be nice to have this added.