Fix setting of fill_value for string columns in Python 3 - #2733

Fix looks OK, two minor points:

• the assert should be changed to np.testing.assert_.
• could you change the test docstring to # Regression test for #2733 (now it hides the test name when running test(verbose=2).

@rgommers - do you mean that it should be:

    def test_set_fill_value_unicode_py3(self):
        """
        # Regression test for #2733
        """

or

    def test_set_fill_value_unicode_py3(self):
        # Regression test for #2733

The way I had it was the same as for the other regression tests, and the output with verbose=2 is currently:

Ticket #1559 ... ok
Ticket #61 ... ok
Ticket #17 ... ok
Ticket #254 ... ok
Ticket #271 ... ok
Ticket #1256 ... ok
Ticket #62 ... ok
Ticket #2733 ... ok

for the test_regression.py file, so changing just the test in this PR would make it inconsistent with the others. What do you think?

I meant the latter. But if that file has many similar issues, it's fine to leave it as is now. I'll fix it once I get annoyed again by not being able to find some failing test quickly.

OK, merged. Thanks @astrofrog

@certik any objection to backporting this?

No objections.