gh-115035: Mark ThreadHandles as non-joinable earlier after forking (#115042)

This marks dead ThreadHandles as non-joinable earlier in
`PyOS_AfterFork_Child()` before we execute any Python code. The handles
are stored in a global linked list in `_PyRuntimeState` because `fork()`
affects the entire process.

Python/pystate.c

@@ -517,6 +517,8 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
         return _PyStatus_NO_MEMORY();
     }
 
+    _PyThread_AfterFork(&runtime->threads);
+
     return _PyStatus_OK();
 }
 #endif