BUG: Use astimezone instead of tz_convert.

Makes get_datetime agnostic as to whether its input is a datetime or a
Timestamp.  (astimezone is an alias to tz_convert when the input is a
Timestamp).

zipline/algorithm.py

@@ -784,7 +784,7 @@ class TradingAlgorithm(object):
         assert date_copy.tzinfo == pytz.utc, \
             "Algorithm should have a utc datetime"
         if tz is not None:
-            date_copy = date_copy.tz_convert(tz)
+            date_copy = date_copy.astimezone(tz)
         return date_copy
 
     def set_transact(self, transact):