Add `Portal.chan` property, to wrap `._chan` attr

tractor/_portal.py

@@ -97,7 +97,7 @@ class Portal:
         channel: Channel,
     ) -> None:
 
-        self.chan = channel
+        self._chan: Channel = channel
         # during the portal's lifetime
         self._final_result_pld: Any|None = None
         self._final_result_msg: PayloadMsg|None = None
@@ -109,6 +109,10 @@ class Portal:
         self._streams: set[MsgStream] = set()
         self.actor: Actor = current_actor()
 
+    @property
+    def chan(self) -> Channel:
+        return self._chan
+
     @property
     def channel(self) -> Channel:
         '''