gh-76785: Make interpreters.*Channel Objects Shareable (gh-110607)

This restores their shareability, which was disabled by gh-110318 due to ref leaks.

Author: ericsnowcurrently

Lib/test/support/interpreters.py

@@ -264,4 +264,4 @@ def close(self):
 
 
 # XXX This is causing leaks (gh-110318):
-#_channels._register_end_types(SendChannel, RecvChannel)
+_channels._register_end_types(SendChannel, RecvChannel)

Lib/test/test_interpreters.py

@@ -833,7 +833,6 @@ def test_list_all(self):
         after = set(interpreters.list_all_channels())
         self.assertEqual(after, created)
 
-    @unittest.expectedFailure  # See gh-110318:
     def test_shareable(self):
         rch, sch = interpreters.create_channel()
 

Modules/_xxinterpchannelsmodule.c

@@ -441,6 +441,10 @@ _get_current_module_state(void)
 static int
 traverse_module_state(module_state *state, visitproc visit, void *arg)
 {
+    /* external types */
+    Py_VISIT(state->send_channel_type);
+    Py_VISIT(state->recv_channel_type);
+
     /* heap types */
     Py_VISIT(state->ChannelInfoType);
     Py_VISIT(state->ChannelIDType);