Deprecate macos close handler.

This is similar to the `_widgetclosed` handler in qt and the `_on_close`
handler in wx.

Author: anntzer

doc/api/next_api_changes/deprecations/23464-AL.rst

@@ -0,0 +1,2 @@
+``FigureManagerMac.close`` is deprecated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

lib/matplotlib/backends/backend_macosx.py

@@ -158,10 +158,14 @@ def __init__(self, canvas, num):
             self.show()
             self.canvas.draw_idle()
 
-    def close(self):
+    def _close_button_pressed(self):
         Gcf.destroy(self)
         self.canvas.flush_events()
 
+    @_api.deprecated("3.6")
+    def close(self):
+        return self._close_button_pressed()
+
     def show(self):
         if not self._shown:
             self._show()

src/_macosx.m

@@ -1132,7 +1132,7 @@ - (NSRect)constrainFrameRect:(NSRect)rect toScreen:(NSScreen*)screen
 
 - (BOOL)closeButtonPressed
 {
-    gil_call_method(manager, "close");
+    gil_call_method(manager, "_close_button_pressed");
     return YES;
 }