[Biopython-dev] docstring tests

[Peter]

> There is also one failure with Python 2.3 which does not test docstrings:
>
> ======================================================================
> ERROR: Test Nexus module
> ----------------------------------------------------------------------
> Traceback (most recent call last):
>  File "test_Nexus.py", line 92, in test_NexusTest1
>   self.assertTrue('codons' in n.charpartitions)
> AttributeError: 'NexusTest1' object has no attribute 'assertTrue'

That is because the unittest assertTrue is only available on python
2.4+, so we should add a quick workaround with a note that this can be
simplified once we drop Python 2.3 support.

Peter