Tests don't work on Python 2.6

[Michał Górny]

ERROR: test_format_registration (pymaging.tests.test_basic.DrawTests)
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Traceback (most recent call last):
  File "/home/mgorny/git/pymaging/pymaging/tests/test_basic.py", line 301, in test_format_registration
    self.assertIsInstance(img, Image)
AttributeError: 'DrawTests' object has no attribute 'assertIsInstance'

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The assertIsInstance() function is not available in Python 2.6. Possible solutions:

1. Use unittest2.TestCase as base class in Python 2.6,
2. Re-implement it in pymaging in Python 2.6,
3. Replace its use with assertTrue(isinstance(...)) or something similar.

[Jonas Obrist]

Thanks for reporting, fixed.