feat: set transport to none after shutdown

Author: mitsuhiko

sentry_sdk/client.py

@@ -148,6 +148,7 @@ def close(self, timeout=None, shutdown_callback=None):
             if timeout is None:
                 timeout = self.options["shutdown_timeout"]
             self.transport.shutdown(timeout=timeout, callback=shutdown_callback)
+            self.transport = None
 
     def __enter__(self):
         return self