fix set_positions

Author: ianhi

mpl_point_clicker/_clicker.py

@@ -123,7 +123,7 @@ def set_positions(self, positions):
             if k not in self._classes:
                 raise ValueError(f"class {k} is not in {self._classes}")
 
-        for k, v in positions.keys():
+        for k, v in positions.items():
             self._positions[k] = list(v)
         self._observers.process('pos-set', self.get_positions())