Reflect matplotlib switch from legend.legendHandles to legend.legend_…

…handles

src/tikzplotlib/_util.py

@@ -12,7 +12,7 @@ def get_legend_text(obj):
     if leg is None:
         return None
 
-    keys = [h.get_label() for h in leg.legendHandles if h is not None]
+    keys = [h.get_label() for h in leg.legend_handles if h is not None]
     values = [t.get_text() for t in leg.texts]
 
     label = obj.get_label()