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

The _Py_IsMainInterpreter() function now expects interp rather than
tstate.
diff --git a/Python/pystate.c b/Python/pystate.c
index 922e5be..f4fd039 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1327,7 +1327,7 @@ PyThreadState_IsCurrent(PyThreadState *tstate)
 PyStatus
 _PyGILState_Init(PyThreadState *tstate)
 {
-    if (!_Py_IsMainInterpreter(tstate)) {
+    if (!_Py_IsMainInterpreter(tstate->interp)) {
         /* Currently, PyGILState is shared by all interpreters. The main
          * interpreter is responsible to initialize it. */
         return _PyStatus_OK();