TST: Fix test_multiprocessing_windows_spawn on recent Pandas

Fixes error:

> TypeError: Index(...) must be called with a collection of some kind, '[^_]' was passed

Author: kernc

backtesting/test/_test.py

@@ -611,7 +611,7 @@ def test_multiprocessing_windows_spawn(self):
                 stats2 = Backtest(df, SmaCross).optimize(**kw)
 
         self.assertIn('multiprocessing support', cm.warning.args[0])
-        assert stats1.filter('[^_]').equals(stats2.filter('[^_]')), (stats1, stats2)
+        assert stats1.filter(chars := tuple('[^_]')).equals(stats2.filter(chars)), (stats1, stats2)
 
     def test_optimize_invalid_param(self):
         bt = Backtest(GOOG.iloc[:100], SmaCross)