asyncio: Cleanup logging in BaseEventLoop._run_once()

logger.log() is now responsible to format the timeout. It might be faster if
the log is disabled for DEBUG level, but it's also more readable and fix
an issue with Python 2.6 in the Trollius project.

Lib/asyncio/base_events.py

@@ -614,12 +614,15 @@ class BaseEventLoop(events.AbstractEventLoop):
             t0 = self.time()
             event_list = self._selector.select(timeout)
             t1 = self.time()
-            argstr = '' if timeout is None else ' {:.3f}'.format(timeout)
             if t1-t0 >= 1:
                 level = logging.INFO
             else:
                 level = logging.DEBUG
-            logger.log(level, 'poll%s took %.3f seconds', argstr, t1-t0)
+            if timeout is not None:
+                logger.log(level, 'poll %.3f took %.3f seconds',
+                           timeout, t1-t0)
+            else:
+                logger.log(level, 'poll took %.3f seconds', t1-t0)
         else:
             event_list = self._selector.select(timeout)
         self._process_events(event_list)