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

The _Py_IsMainInterpreter() function now expects interp rather than
tstate.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 64fd408..498f393 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -15701,7 +15701,7 @@ _PyUnicode_Init(PyThreadState *tstate)
         return _PyStatus_NO_MEMORY();
     }
 
-    if (_Py_IsMainInterpreter(tstate)) {
+    if (_Py_IsMainInterpreter(tstate->interp)) {
         /* initialize the linebreak bloom filter */
         bloom_linebreak = make_bloom_mask(
             PyUnicode_2BYTE_KIND, linebreak,