apply patch from #2349.  replaces use of TestCase.assertIsInstance(x, y) with TestCase.assertTrue(isinstance(x, y)).  author: moe, reviewer: exarkun.  closes #2349.
apply patch from #2349.  replaces use of TestCase.assertIsInstance(x, y) with TestCase.assertTrue(isinstance(x, y)).  author: moe, reviewer: exarkun.  closes #2349.

twisted/mantissa,twisted/mantissa,twisted/mantissa