fix pep8

Author: Shlomi Kushchi

alpaca_trade_api/entity.py

@@ -128,6 +128,7 @@ def df(self):
                 self._df = pd.concat(dfs, axis=1)
         return self._df
 
+
 class _Timestamped(object):
     def __getattr__(self, key):
         if key in self._raw:
@@ -145,6 +146,7 @@ class _NanoTimestamped(_Timestamped):
 class _MilliTimestamped(_Timestamped):
     unit = 'ms'
 
+
 class Agg(_MilliTimestamped, Entity):
     pass