Use reason_phrase property rather than non-public attribute

Author: tim-mccurrach

rest_framework/response.py

@@ -49,7 +49,7 @@ def __init__(
         self.exception = exception
         self.content_type = content_type
         if reason:
-            self._reason_phrase = f"{self.status_text} ({reason})"
+            self.reason_phrase = f"{self.status_text} ({reason})"
 
         if headers:
             for name, value in headers.items():