Fix type doc for scroll event "step" attribute.

anntzer · anntzer · commit 248c0839a567 · 2020-06-23T11:29:02.000+02:00
`step` is a float e.g. on Qt; this is intentional as
"some mice have finer-resolution wheels and send delta values that are
less than 120 units" (quoting the Qt docs).
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -1403,7 +1403,7 @@ class MouseEvent(LocationEvent):
            last change of keyboard state occurred while the canvas did not have
            focus, this attribute will be wrong.
 
-    step : int
+    step : float
         The number of scroll steps (positive for 'up', negative for 'down').
         This applies only to 'scroll_event' and defaults to 0 otherwise.
 
