REF: Fix pandas deprecation warning

> FutureWarning: casting datetime64[ns] values to int64
> with .astype(...) is deprecated and will raise in a future
> version. Use .view(...) instead.

Author: kernc

backtesting/_plotting.py

@@ -138,10 +138,10 @@ def _weighted_returns(s, trades=trades):
         return ((df['Size'].abs() * df['ReturnPct']) / df['Size'].abs().sum()).sum()
 
     def _group_trades(column):
-        def f(s, new_index=df.index.astype(np.int64), bars=trades[column]):
+        def f(s, new_index=pd.Index(df.index.view(int)), bars=trades[column]):
             if s.size:
                 # Via int64 because on pandas recently broken datetime
-                mean_time = int(bars.loc[s.index].view('i8').mean())
+                mean_time = int(bars.loc[s.index].view(int).mean())
                 new_bar_idx = new_index.get_loc(mean_time, method='nearest')
                 return new_bar_idx
         return f