Simplify curvezmq ctx destructor (#3050)

The underlying `zmq.Context` object has its own `__del__` method, which
calls `self.destroy()`, so we only need to close the auth thread.

parsl/curvezmq.py

@@ -49,9 +49,6 @@ def __init__(self, cert_dir: Optional[Union[str, os.PathLike]]) -> None:
         self.cert_dir = cert_dir
         self._ctx = zmq.Context()
 
-    def __del__(self):
-        self.destroy()
-
     @property
     def encrypted(self):
         """Indicates whether encryption is enabled.
@@ -136,6 +133,12 @@ def __init__(self, cert_dir: Optional[Union[str, os.PathLike]]) -> None:
         if self.encrypted:
             self.auth_thread = self._start_auth_thread()
 
+    def __del__(self):
+        # Avoid issues in which the auth_thread attr was
+        # previously deleted
+        if getattr(self, "auth_thread", None):
+            self.auth_thread.stop()
+
     def _start_auth_thread(self) -> ThreadAuthenticator:
         auth_thread = ThreadAuthenticator(self._ctx)
         auth_thread.start()