When re-opening a ResumableBidiRPC set _request_queue_generator to None. (#7548)

Author: crwilcox

google/api_core/bidi.py

@@ -350,7 +350,7 @@ def _reopen(self):
             self.call = None
             # Request generator should exit cleanly since the RPC its bound to
             # has exited.
-            self.request_generator = None
+            self._request_generator = None
 
             # Note: we do not currently do any sort of backoff here. The
             # assumption is that re-establishing the stream under normal