Add type annotation for LocationEvent.modifiers (matplotlib#30841)

Author: NMertsch

lib/matplotlib/backend_bases.py

@@ -1262,7 +1262,7 @@ class LocationEvent(Event):
     xdata, ydata : float or None
         Data coordinates of the mouse within *inaxes*, or *None* if the mouse
         is not over an Axes.
-    modifiers : frozenset
+    modifiers : frozenset[str]
         The keyboard modifiers currently being pressed (except for KeyEvent).
     """
 

lib/matplotlib/backend_bases.pyi

@@ -237,6 +237,7 @@ class LocationEvent(Event):
     inaxes: Axes | None
     xdata: float | None
     ydata: float | None
+    modifiers: frozenset[str]
     def __init__(
         self,
         name: str,