RadioButtons: fix self._clicked method (followup to matplotlib#30997)

In matplotlib#30997 the classes `RadioButtons` & `CheckButtons` started sharing
more code, as they are fundamentally similar. When copy-pasting the
methods that were seemingly identical, the `_clicked` method was copied
from the original `CheckButtons` class, and there the `self._frames`
object was used instead of `self._buttons`. This caused an error when
actually using and clicking on buttons created with `RadioButtons`, as
the `RadioButtons._frames` doesn't exist - something that unfortunately
the tests did not catch.

Both `CheckButtons._frames` and `CheckButtons._buttons` are very similar
so even before matplotlib#30997 the `CheckButtons._clicked` method could have used
`self._checks` and not `self._frames`. Hence this change should be
harmless.

Author: doronbehar

lib/matplotlib/widgets.py

@@ -1110,11 +1110,11 @@ def _clicked(self, event):
         if self.ignore(event) or event.button != 1 or not self.ax.contains(event)[0]:
             return
         idxs = [  # Indices of frames and of texts that contain the event.
-            *self._frames.contains(event)[1]["ind"],
+            *self._buttons.contains(event)[1]["ind"],
             *[i for i, text in enumerate(self.labels) if text.contains(event)[0]]]
         if idxs:
-            coords = self._frames.get_offset_transform().transform(
-                self._frames.get_offsets())
+            coords = self._buttons.get_offset_transform().transform(
+                self._buttons.get_offsets())
             self.set_active(  # Closest index, only looking in idxs.
                 idxs[(((event.x, event.y) - coords[idxs]) ** 2).sum(-1).argmin()])