STY: remove unused imports and method, clean up docs

tests/pipeline/test_classifier.py

@@ -520,7 +520,7 @@ class TestPostProcessAndToWorkSpaceValue(ZiplineTestCase):
             missing_value = -1
 
         f = F()
-        column_data =np.array(
+        column_data = np.array(
             [[0, f.missing_value],
              [1, f.missing_value],
              [2, 3]],

tests/pipeline/test_engine.py

@@ -34,7 +34,7 @@ from pandas import (
 from pandas.compat.chainmap import ChainMap
 from pandas.util.testing import assert_frame_equal
 from six import iteritems, itervalues
-from toolz import merge, assoc
+from toolz import merge
 
 from zipline.assets.synthetic import make_rotating_equity_info
 from zipline.errors import NoFurtherDataError