mark _thread.interrupt_main signature unsupported

previously IntEnum was rendered as 2.  I think we should
rather preserve repr for int's subclass, like pure-Python
methods do.

see also #61005

Author: skirpichev

Lib/test/test_inspect/test_inspect.py

@@ -6375,7 +6375,9 @@ def test_threading_module_has_signatures(self):
     def test_thread_module_has_signatures(self):
         import _thread
         no_signature = {'RLock'}
-        self._test_module_has_signatures(_thread, no_signature)
+        unsupported_signature = {'interrupt_main'}
+        self._test_module_has_signatures(_thread, no_signature,
+            unsupported_signature=unsupported_signature)
 
     def test_time_module_has_signatures(self):
         no_signature = {