Change Exit.__init__ params order to keep backward compatibility

src/_pytest/outcomes.py

@@ -49,7 +49,7 @@ class Failed(OutcomeException):
 class Exit(KeyboardInterrupt):
     """ raised for immediate program exits (no tracebacks/summaries)"""
 
-    def __init__(self, returncode=None, msg="unknown reason"):
+    def __init__(self, msg="unknown reason", returncode=None):
         self.msg = msg
         self.returncode = returncode
         KeyboardInterrupt.__init__(self, msg)