Deprecate setting the timer interval while starting it.

This feature is unused internally and has simple replacements; removing
it will make it easier for subclasses to directly override Timer.start
rather than having Timer.start handle the interval and then deferring to
a private _timer_start.

Author: anntzer

doc/api/next_api_changes/deprecations/26894-AL.rst

@@ -0,0 +1,6 @@
+*interval* parameter of ``TimerBase.start``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Setting the timer *interval* while starting it is deprecated.  The interval can
+be specified instead in the timer constructor, or by setting the
+``timer.interval`` attribute.

lib/matplotlib/backend_bases.py

@@ -1117,6 +1117,7 @@ def __del__(self):
         """Need to stop timer and possibly disconnect timer."""
         self._timer_stop()
 
+    @_api.delete_parameter("3.9", "interval", alternative="timer.interval")
     def start(self, interval=None):
         """
         Start the timer object.