py.test fixture used in #1633 causes problems

[ctb]

So @betatim introduced this:

    @pytest.fixture
    def reads():
        infile = utils.get_test_data('valid-read-testing.fq')
        reads = ReadParser(infile)
        yield reads
        reads.close()

which causes problems on my mac, which is using pytest 2.9.1:

/Users/t/dev/khmer/tests/test_sequence_validation.py:70
____ ERROR at setup of test_read_cleaning_trim_functions_bad_dna[Nodegraph] ____
pytest.fixture functions cannot use ``yield``. Instead write and return an inner function/generator and let the consumer call and iterate over it.:

I have switched to using a try/finally; this issue is here to tell @betatim why.

[ctb]

ref #1633

[betatim]

Oups! I'm on py.test 3 already. In 2.9 it should be @pytest.yield_fixture http://doc.pytest.org/en/2.9.2/fixture.html#fixture-finalization-executing-teardown-code