REF: Fix pandas deprecation warning (#725)

* REF: Fix pandas deprecation warning

> FutureWarning: Passing method to Int64Index.getloc
> Use index.get_indexer([item], method=...) instead.

* Update backtesting/_plotting.py

Author: kawishqayyum

backtesting/_plotting.py

@@ -145,7 +145,7 @@ 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(int).mean())
-                new_bar_idx = new_index.get_loc(mean_time, method='nearest')
+                new_bar_idx = new_index.get_indexer([mean_time], method='nearest')[0]
                 return new_bar_idx
         return f