Fix read-only error triggered by Pandas 2.3 Copy-on-Write

Author: edtechre

src/pybroker/common.py

@@ -379,5 +379,5 @@ def get_unique_sorted_dates(col: pd.Series) -> Sequence[np.datetime64]:
     # TODO: Remove after Pandas 1.0 is no longer supported.
     if hasattr(result, "to_numpy"):
         result = result.to_numpy()
-    result.sort()
+    result = np.sort(result)
     return result

src/pybroker/strategy.py

@@ -1327,8 +1327,7 @@ def _run_walkforward(
                         DataCol.DATE.value
                     ].values
                     if len(sym_dates):
-                        sym_dates.sort()
-                        exit_dates[sym] = sym_dates[-1]
+                        exit_dates[sym] = sym_dates.max()
         signals: dict[str, pd.DataFrame] = {}
         for train_idx, test_idx in self.walkforward_split(
             df=df,