MAINT: Use explicit relative pathing for Python 3 compatibility.

Python 3 requires submodules to have more explicit pathing, so use
the dot syntax to declare submodules which are in the same directory
as another module.

tests/risk/test_risk_cumulative.py

@@ -23,7 +23,7 @@ from zipline.utils import factory
 
 from zipline.finance.trading import SimulationParameters
 
-import answer_key
+from . import answer_key
 ANSWER_KEY = answer_key.ANSWER_KEY
 
 

zipline/finance/risk/period.py

@@ -26,7 +26,7 @@ from zipline.finance import trading
 
 import pandas as pd
 
-import risk
+from . import risk
 from . risk import (
     alpha,
     check_entry,