gh-128212: Fix race in _PyUnicode_CheckConsistency (GH-128367)

There was a data race on the utf8 field between `PyUnicode_SET_UTF8` and
`_PyUnicode_CheckConsistency`. Use the `_PyUnicode_UTF8()` accessor,
which uses an atomic load internally, to avoid the data race.

Objects/unicodeobject.c

@@ -688,7 +688,7 @@ _PyUnicode_CheckConsistency(PyObject *op, int check_content)
                                  || kind == PyUnicode_2BYTE_KIND
                                  || kind == PyUnicode_4BYTE_KIND);
             CHECK(ascii->state.ascii == 0);
-            CHECK(compact->utf8 != data);
+            CHECK(_PyUnicode_UTF8(op) != data);
         }
         else {
             PyUnicodeObject *unicode = _PyUnicodeObject_CAST(op);