Use more specific assertIsInstance in tests (#268)

When running tests with Python warnings enabled, warnings of the
following form appear:

DeprecationWarning: Please use assertTrue instead.
  self.failUnless(isinstance(...))

Use assertIsInstance instead to fix these warnings.

Using a more specific assert also has the advantage of more informative
error reporting upon failure.

https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertTrue

> This method should also be avoided when more specific methods are
> available (e.g. assertEqual(a, b) instead of assertTrue(a == b)),
> because they provide a better error message in case of failure.

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