Add RotatingFileHandler.__del__ that closes stream

Eliminates 5 of the `ResourceWarning` warnings mentioned in GH-391.

supervisor/loggers.py

@@ -184,6 +184,13 @@ class RotatingFileHandler(FileHandler):
         self.counter = 0
         self.every = 10
 
+    def __del__(self):
+        if self.stream:
+            try:
+                self.stream.close()
+            except OSError as exc:
+                pass
+
     def emit(self, record):
         """
         Emit a record.