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

The _Py_IsMainInterpreter() function now expects interp rather than
tstate.
diff --git a/Python/import.c b/Python/import.c
index 75ac21d..6189dcf 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -441,7 +441,7 @@ _PyImport_FixupExtensionObject(PyObject *mod, PyObject *name,
         return -1;
     }
 
-    if (_Py_IsMainInterpreter(tstate)) {
+    if (_Py_IsMainInterpreter(tstate->interp)) {
         if (def->m_size == -1) {
             if (def->m_base.m_copy) {
                 /* Somebody already imported the module,