Fix removal of Figure-level artists

PyCharm has been warning that `Artist.figure` should be a `Figure|None`,
not a `bool` as is set in this block.

Based on the previous section of code, I think setting the figure to
stale (if the Axes was not already set stale, meaning the Artist was on
a Figure *only*) was the intended behaviour.

Author: QuLogic

lib/matplotlib/artist.py

@@ -249,9 +249,9 @@ def remove(self):
                 _ax_flag = True
 
             if self.figure:
-                self.figure = None
                 if not _ax_flag:
-                    self.figure = True
+                    self.figure.stale = True
+                self.figure = None
 
         else:
             raise NotImplementedError('cannot remove artist')