Fix the last failing test in Python 3

In my original "support Python 3" pull request, I mentioned a [strange unit test failure](https://github.com/brentp/pyfasta/issues/9#issuecomment-17244403) that I didn't fix at the time. This seemed like a good time to look in to this and fix it. The `pyfasta` code was fine; the test was what was broken. Everything looks great for Python 3 now but I don't have an environment set up to test this on Python 2, so if the tests pass for you I think it's good to go.

Output of `test_all.py`:
```
Ran 102 tests in 1.840s

OK
```