bpo-43268: _Py_IsMainInterpreter() now expects interp (GH-24577)

The _Py_IsMainInterpreter() function now expects interp rather than
tstate.
diff --git a/Python/context.c b/Python/context.c
index 82826bf..6a45621 100644
--- a/Python/context.c
+++ b/Python/context.c
@@ -1302,7 +1302,7 @@ _PyContext_ClearFreeList(PyThreadState *tstate)
 void
 _PyContext_Fini(PyThreadState *tstate)
 {
-    if (_Py_IsMainInterpreter(tstate)) {
+    if (_Py_IsMainInterpreter(tstate->interp)) {
         Py_CLEAR(_token_missing);
     }
     _PyContext_ClearFreeList(tstate);