Fix slow_callback_duration repr in warnings for TimerHandles

Issue #103.

tests/test_base.py

@@ -348,8 +348,8 @@ class _TestBase:
         # format message
         msg = log.call_args[0][0] % log.call_args[0][1:]
 
-        # self.assertIn('Executing <Handle', msg)
-        # self.assertIn('test_debug_slow_callbacks', msg)
+        self.assertIn('Executing <TimerHandle', msg)
+        self.assertIn('test_debug_slow_timer_callbacks', msg)
 
     def test_default_exc_handler_callback(self):
         self.loop._process_events = mock.Mock()

uvloop/cbhandles.pyx

@@ -166,7 +166,6 @@ cdef class TimerHandle:
         if self.timer is None:
             return
 
-        self.callback = None
         self.args = None
 
         try: