Fix flaky test on python 2.7

Something is killing the skein daemon between tests on python 2.7 only.
I've determined it's not a bug in:
- Skein (close/del is implemented properly and tested in skein)
- dask-yarn (finalizers aren't being called)
- py.test (daemon persists between tests if dask isn't used)

It may be a weird interaction between py.test and signal handling that
distributed/tornado sets up, idk. Since python 2.7 is going away
shortly, I'm just going to punt on this. For now we just create a new
client for each test and things work fine.

dask_yarn/tests/test_core.py

@@ -25,7 +25,7 @@ def conda_env():
     return envpath
 
 
-@pytest.fixture(scope='module')
+@pytest.fixture
 def skein_client():
     with skein.Client() as client:
         yield client