fix: address type hints for _extract_line_data method in MultiLinePlot (#152)

Author: dakshpokar

Diff for: maidr/core/plot/lineplot.py

@@ -1,3 +1,5 @@
+from typing import List, Union
+
 from matplotlib.axes import Axes
 from matplotlib.lines import Line2D
 
@@ -59,7 +61,9 @@ def _extract_plot_data(self) -> list[dict]:
 
         return data
 
-    def _extract_line_data(self, plot: list[Line2D] | None) -> list[dict] | None:
+    def _extract_line_data(
+        self, plot: Union[List[Line2D], None]
+    ) -> Union[List[dict], None]:
         """
         Extract data from multiple line objects.