bpo-38781: Clear buffer in MemoryHandler flush (GH-17132)

This makes it easier to use a custom buffer when subclassing
MemoryHandler (by avoiding the explicity empty list literal
assignment in the flush method). For example, collection.deque
can now be used without any modifications to MemoryHandler.flush.

The same applies to BufferingHandler.

Lib/logging/handlers.py

@@ -1254,7 +1254,7 @@ def flush(self):
         """
         self.acquire()
         try:
-            self.buffer = []
+            self.buffer.clear()
         finally:
             self.release()
 
@@ -1321,7 +1321,7 @@ def flush(self):
             if self.target:
                 for record in self.buffer:
                     self.target.handle(record)
-                self.buffer = []
+                self.buffer.clear()
         finally:
             self.release()